At first glance at my question, I probably sound like a horrible person. I assure you I'm not... I'm so confused.
Here's my story...
I've been dating my fiancé (we'll call him Adam) for about 11 years. During those 11 years I broke it off with him because I felt like I wasn't in love with him and I just lost that feeling of wanting him. During the time we broke up, I dated different guys and kept finding myself comparing them to Adam... and realizing, I really can't find anyone with the compatibility I have with him. So about a year later, Adam and I talked about it and wound up getting back together... I was figuring to myself... If I keep comparing all these other guys to Adam, then I should be with Adam. He's "the one" and I'm just too dense to realize this...
So back in June Adam proposes to me... I was TOTALLY taken off guard. I was under the impression we were going to move in with each other first... or at least that's what we have said... regardless, I said yes and I was excited... A month after Adam proposes, I'm playing around on Facebook and find an old friend I haven't seen in like 12 years (we'll call him Zach). We emailed back and forth catching up on time past... Then we decided to meet up for a drink. We had a nice night... Very friendly. After we hang out we continue to email... then we decide to grab a drink again... this time was different. I felt an attraction and we wound up kissing by the end of the night. Next thing I know, we are wanting to see each other all the time... I think I'm falling for him. It's crazy. He's constantly on my mind. I've never felt this way about a guy. This has been going on for months now so I've finally told Zach we need to back off so I can wrap my head around all this and figure out what I need to do...
So now... here is my dilemma... I'm engaged and have found that loving and wanting feeling with another. Because Adam and I have been together for so long, we have all the same friends... our families love us dearly... My friends are going to hate me and family will be disappointed... The worst part is... if I do break up with Adam, how do I know if I am making a huge mistake? How do I break up my engagement? What's the best way to go about this? Do I just try to work things out with Adam?

It sounds like you may be just settling with Adam. It could be best for you to break up with him so that the both of you could find someone that you would be happier with. It sounds like you're happier with Zach, and maybe it would be better if you were with him.

Thank you for your response teastalk... I realize there is no easy way to break off an engagement, but my question is, what is the best way to go about it in my situation... Adam is such a great guy so I really don't want to add salt to the wound and tell him I've fell in love with another guy... How do I do this?

I agree with you. There is no reason to give him additional pain by letting him know that you have found someone else. Perhaps sit down and let him know what you feel.

May be just tell him basically what you said here except without the part concerning Zach.

You need to talk honestly with Adam and break it off. Sounds like you’ve been unhappy in the relationship for a while, so why drag it out? It’ll be painful for him to hear the truth, but he deserves to hear it straight from you. You can’t worry about what your friends and family think - it’s your life.

Adam doesn’t deserve being treated with lies and deceit. It just isn’t fair to Adam for you to be sneaking around with other men, so just do it quickly and let him move on with his life. Once you’re free you can date all the men you’d like without feeling guilty.

Ok!
1)If you breakup with Adam don't do it because of this other guy.This is new and it is normal to be excited by something new but this is surely not love.
You have to be able to make the difference between a flamme and an eternal fire.
2)If you doubt that much for a guy you met a few times just stop the relationship with Adam.
3)Please you're saying Adam is a good guy!You should be able to respect this guy you know for 11 years and tell him all the truth cause if you leave him like that he will keep asking himself what did I do wrong?
He didn't do something wrong ! You want to be with another you need to assume it even if it's hard.
He doesn't deserve lies.

Why did you say yes?

That's the real question.

You should get your priorities straight. Figure out what it is you want, instead of hurting people. Then get into relationships. Get in tune with yourself.

Guilty? Not sure its in your vocab.

Seems like you fall in love quickly.

Maybe spend some time single for a while.

Ie: no steady boyfriend.

Figure it out.

After 11 years, I think that he deserves to hear the truth.

Be honest