I was in a similar situation that you were in, I was in a long term relationship, we were engaged, I was beginning to plan my life around the expectation that she would be a part of it and then unexpectedly, at least to me and my friends, she dumped me in a pretty horrible and selfish way. And, much like you, I was utterly crushed and thoroughly scared to jump back into any sort of relationship with women, muchless a romantic one. It took about a year before I felt like I was ready to start dabbling again, and I had many of the problems you’re having. Anyway, let me give you some advice based on the lessons I’ve learned since that break-up and the subsequent months of dating since I’ve been back in it.
First, dating a girl doesn’t necessarily mean a “relationship”. In fact, it doesn’t really mean anything except that you spent time with her. As others have said upthread, you’re putting a lot of expectations on a relationship when you think about it and treat it that way. You shouldn’t approach every girl you meet with a “am I going to spend the rest of my life with her” sort of mentality, and doing so is akin to a kindergartener being concerned with what college they’re going to go to when they graduate from high school. Go on a date, spend time with her, try to have fun, and let things happen naturally. She may be a long term girlfriend or even spouse, she may be someone you date a couple times and decide to just be friends, you may not get along in the long term at all, but with high expectations, it’s hard not to be crushed if they aren’t met.
Second, and there really isn’t a nice way to say it, but work on your baggage and you may even want to seek some help with it. Carrying issues from one relationship to another is just asking for heartache and, in my opinion, it just isn’t fair to whoever comes next. The fact that she has shown interest despite you displaying these sorts of signs may ultimately betray a set of issues that she may have yet to deal with as well. However, I commend you for being honest about it with her because at least she’s able to evaluate the situation and decide for herself if she’s willing and able to deal with them as you work on them (or possibly don’t).
Also, don’t worry about messing things up, that’s what the whole dating process is for and why I’ve suggested not trying to have those high expectations so you don’t get overly emotionally attached. Instead, I liken dating to a job interview. Sure, if you go in to interview for a job with the expectation that you’ll work there for the next 40 years until you retire, of course you’ll be heartbroken if it doesn’t work out. On the other hand, if you go in thinking it may be a good opportunity and it pans out, great, and if it doesn’t, not a big deal. That is, you might go on a date, and it’s not unlikely that you’ll mess up somehow. Unless you did something horribly wrong like smack her or deliberately insult her, you’re not going to hurt her if things don’t work out on the first few dates because, hopefully, she hasn’t overly invested her emotions either.
So, really, just relax, have fun, be yourself, and go from there.