I have been dating the guy for almost 6 months, everythings been great until a few days ago. I started questioning but the past months I never questioned my feelings I was positive I was in love with him. Now one minute I will know I love him and then the next I'm not so sure. Every time I think about it I start to cry histerically because he's the best thing that's ever happened to me, and the only one that understands me completely. I just don't understand how this could change so fast. What should I do?

After some time the intense 'I'm in love' emotions often change into something more calm and settled.
Could this be the case?

Or are you falling out of love?
Has anything specific happened to make you feel this way?

Are you bored? DO you two go out much? When you feel you don't love him, what goes on your head that you are having these feelings? IS the relationship at a stand still? Not going anywhere? Are you wanting more from a relationship? Do you see yourself marrying this person? Is he a provider? A go getter?
What's triggering the out of love?

I second those questions for sure. And Add some.
Did he do something that is making you doubt him? Do you find yourself attracted to another?" and honestly it has been 6 months, I know you love him but if a break is to happen it is probably best this early on so it hurts less. Believe me a break after 1.5 years is way too much to handle when you believe the person is your "soulmate"

Figure out how you feel and why you feel that way, you both deserve the conclusion sooner rather than later whether its staying together and talking about the feeling you had been having or breaking up as gently as possible. If you do break up with him though, don't set his car on fire lol. I know someone who did that and got locked up for it. Not a pretty site.
Good luck

Fear of commitment or fear that the relationship will fail. You do not want to ruin it but at the same time you have started to doubt. Which this is in my opinion your own fears and or questions that have nothing to do with the relationship in itself. You say it has been great and wonderful. So the issue is you and your fear of something happening, or is it you feel you do not deserve such a good relationship for some reason.

I would like to ask what happened as well? Did you have an experience with someone else? Is that why your questioning things. There are so many reasons for this to happen.

Please let us know what is truly going on with YOU?

You don't give your age,teenage relationships tend to become intense very fast,with a lot of input from other teens,most teen relationships burn out quite fast,it's a learning relationship experience.

If you're an adult,fear can set in if you feel insecure and one or both people have poor communication skills.

6 months is not a long time for a relationship,your both still learning from each other,and getting to know each other,perhaps your having doubts about a long term relationship with this guy? Mr right now is not always mr long term.

Looks to me like your having more of a problem understanding your own feelings, rather than understanding the relationship.

Perhaps you're thinking about this too much. Your brain can be wrong, trust in what your heart does around him. If it has become too boring then it was just a fling