The "too good for me" excuse is a very complicated one. The thing is, and I'm speaking from personal experience here, if you have met someone who is a little too good for you, you'll know it. Now it's not about them "stealing your thunder" it's more of the fact that it just seems too surreal and unrealistic. To be frank (again I am speaking from experience here), a girl too good for a guy is the sensation of the guy feeling unneeded for the girl. I can imagine many girls feel the same about this. I wouldn't say this is playing on insecurities exactly, it's more or less challenging one's personal thoughts on social expectations, and to some degree their common sense. For me personally I think it's on the level of knowing the compatibility me and a potential partner may have which isn't necessarily bad, but, it just feels like they deserves more. And I guess that's the real problem right there, I feel like I'll be spoiled with something too good for me while they'll have to "put up" with me. Usual trait patterns are when you seem to realize you would screw things up too many times, cheat and get away with it or not being able to reciprocate feelings efficiently enough.
Overall he may not exactly feel guilty to be in a relationship with you but he surely doesn't think you being with him is ideal for you. I know this can sound a little offensive and patronising since you should be the judge of that and not him. But you haven't really given him much to believe he deserves anything from you which is why this has happened. It may or may not be too late for him and you, I do not know because you haven't really explained any encounter that seems to have progressed your situation or make it worse.
I will say though, in terms of self-esteem, you shouldn't just feel bad for no reason. Do you think yourself being overweight is a problem, not necessarily lowering the guys that take an interest in you, but an actual problem? You should first and foremost get into grips with what your romantic interests are. Are you interested in seeing men, or just your current crush? Have you got the confidence to approach men of your interest or do you want them to approach you? It seems that you may not really have ventured in to romance all that much, as it takes quite some effort if you don't actively do it often. I completely get how you sense your accomplishments and surprisingly pleasant lifestyle may put others off based on the past, but you need to put a new angled approach which centers mostly upon your personality.