When a guy has so many insecurities about himself... does it lead to him mistrusting people... family members... and his girlfriend? (Even when none have ever given him a reason to mistrust them.)

Yes dear.. And your point is?
You need to give us an example of his distrust and how it happened. Only then, can we give you more accurate advice.
So, try and be a little more specific and we'll be glad to help you out.

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I know someone that is very insecure. They feel like the world is out to get them. They have a hard time trusting people. This is something you are not going to be able to change with this guy.

Okay... an example...
I have a male friend, who I have known since I was a child... and he is extremely close to the rest of my family... he, my best friend (female)and I get together to have lunch every once in a while... my friend asked me to go to a holiday get-to-gether at the home of mutual friends... I got accused of "doing more than going straight home".
On a couple of Friday nights, I met my best friend at high school football games, because her son plays. I got accused of "seeing someone else"... even though he knew I was going to the football games.
I know these are only two examples... there are more... but I have done nothing out of line or inappropriate to earn the distrust or the accusations. I have been totally faithful... I know how to respect the boundries of a relationship...

I just wanted to know if insecurites play any type of major role in a man mistrusting so many people... not just with my situation, with any in general.

Some people have very entrenched ways of dealing with relationship scenarios and people. These problems usually result from a person's personality development, and once those behaviours are set, there is not much that you can do about it. If the problem is all in his head, then it's both a serious problem, and it's his problem.

You need to be totally up front with him, and don't make the mistake of doing things to prove that you're truthful or not deceiving him. In free societies, complete strangers must consider others innocent until they are proven guilty, not the other way around. He has had the privilege of being close to you, and it is completely unfair that he does not extend any good faith towards you when complete strangers on the street do every day.

You need to assert your boundaries here: tell him that you and your family do not deserve his accusations, monitoring or mistrust. You also need to point out that if he continues being suspicious of his partners, it will undermine his future relationships (and probably has undermined many of his past relationships). If this sounds like him, and you're feeling especially nice, it may be worth directing him to seek help from a trained psychologist/ psychiatrist.

Insecurity + Aggression = jealous controlling behavior

Where it comes from may be an interesting research project, but it seldom has much worth in stopping it.

I believe the only way to stop someone from aiming jealous, controlling behaviors at you is to point at them and say, "Your problem, not mine."

HIM: "You're my girlfriend. I have a right to know who you're with and what you're doing."
YOU: "No, not if this is what you mean by it. I have no more patience for this."

HIM: "I know you're looking at other men/cheating on me!"
YOU: (laugh)"I'm no cheater, but I am a quitter. I will quit on YOU if you keep aiming this stuff at me. Tell me you understand."

You and I both know you're not going to stick around forever for this stuff, right? Admit it. This will get so much easier once you can look in the mirror and acknowledge what is coming.

I started looking in the mirror a few months ago... when he accused me the second time of wanting to be with someone else. I took a second look about 6 weeks ago when it happened again... that's when I finally started asserting my boundries... I informed him that I would no longer defend myself, family or friends against his false accusations, that I have never given him any reason to not trust me, that I refuse to live my life based on mental manipulations and that I have been totally faithful in the whole relationship, and that I will not spend the rest of my life in a relationship where there is only one-sided trust. His take on it was.. "I do trust you, I don't trust your friends. Most men always want more than friendship...they just want to *#^* you." After thinking about that, along with other comments he has made, I then realized that it just isn't his lack of trust in me... it's in a lot of people. I didn't "point blank" tell him that he needed help, because I knew that would cause another blow up.. I just said that I although I wanted to, I just couldn't "fix" certain things, that maybe he needed some outside input. Apparently he understood what I was saying...
I got dumped. I'm not happy, but I'm not harsh or bitter... I was cordial and said hello (nothing more) when we accidentally ran into each other... I love the guy with all my heart, but I know that this isn't the kind of relationship that will be healthy for me... I miss him a whole lot... but I don't miss the loads of drama that I think was brought on by the insecurity...

Thank you all for taking the time to give your input...

Glad you see this is his problem to solve, and not yours. You will be okay, just pray he will be too.

Dear Katho.

I know how much it hurts and you will miss him and the memories of the good times before he 'turned' or you would not be the caring human that you are. Now you can wake up each morning with one less large stress-factor in your life, the rest will follow.

During your healing process, please feel free to vent and let this pain take it's course, as it is part of the healing process - stay with us and we will help you through it. TIME is the main factor here and no matter how long it takes for you to regain balance and harmony, we are here for you.

Just like a flower, you too will blossom anew! HAPPY HEALING!

I do know that I will be okay... it's just going to take some time.
I think right now what "rankles" me the most is that I couldn't deal with all of the insecurities, lack of trust, jealousy, drama and accusations. I've dealt with my late husband's death, raising a teenager by myself, working to pay my own bills, and trying not to ask anyone for anything that I didn't need... but I just couldn't deal with that stuff anymore... I think I'd just had enough.
Since going through my husband's death, I know that everything takes it's time. As the days have passed, I have noticed that I am less stressed... I'm sleeping better and not wondering where I went wrong or what I could do to fix it. I do love the guy, I care about him... but I know it's never been and never will be my place to fix what's wrong... that's up to him.
Okay... I'm done venting for now... I'm going to go read my new book! :O)
Thanks! :O)

Okay people... he's made all of these accusations, tried to manipulate me, doesn't trust me... blah,blah,blah...
I received a visit from a family member(his)... and I am totally amazed that I could have been so totally gullible to believe some of the crap he has told me over the course of our relationship! I can't believe I spent all this time giving him my love, sympathy and whatever else he needed... only to find out that he has twisted the stories around to make him look like either the victim, or the perfect guy! The information that was given to me by the family member matched up almost exactly to the stories that the ex had given me... the only difference was that certain things were not fully revealed, or changed by the ex... because the truth would have made him look bad. And yes, I'm extremely MAD about all of this! But I am going to keep this knowledge to myself and learn from it!
I have been and am now enforcing the N/C rules... I've deleted everything that I can think of... I googled myself to see if I came up anywhere. I even googled my user name... and it came up. So I'm going to change it... and everything else I have to so there will be no contact whatsoever! So, I will be back... because I'll need to vent!