I am very insecure and it has had not only a bad affect on my relationship, but on my social life as well. I developed these insecurities after I caught my first husband with another woman and I've really had a hard time getting rid of them.
You are a splendid, complete person. You know that right?
You speak of your fear of your fiance leaving/cheating on you. Are there other insecurities as well?
I know this is going to sound like a lecture and you've probably told yourself the very same things but I think it needs to be stated so you can gain some focus.
The fact your H decided to cheat on you had nothing to do with your value. It may have something to do with the dynamics of your R. (i.e. you weren't meeting his needs and your reasoning could be you were unaware). But his decision to cheat only speaks of his flawed character not of yours. This doesn't define you in any way so please don't try to own that. Sure it may speak of some non-working R behaviors you may have but with a little education those can easily be fixed. Heh, I'm still working on all my bad behaviors.
May I offer a suggestion? If you aren't seeing an IC (individual counselor) could you look into finding one? Or if you are interested I know of a great friend (found her here at MB) who does Life Coaching (I'm using her myself) and could help you a lot with your insecurities.
If you really don't have the resources to look into these than could you at least post your story over on the Emotional Needs section? There are so many great people there that will help you with more than just R issues. You'll get some great insight and tools to help you fell better about yourself which in turn will help in all your Rs.