Join the club!
If you never feel insecure then you could be a psycho or sociopath. So the fact that you’re insecure “to some degree” actually says good things about you. It shows that you care about how others think and feel. Can that be taken to extremes, too? Of course. Someone who has zero self-confidence comes across as an object of pity, and you don’t want to be that guy, either.
Another way to look at this – to consider the object of your affection, anyway – is to recognize that she has at least had high standards for attractiveness in her companions. Okay, but now look at other aspects of those previous companions. Are they decent people overall? Kind, intelligent, patient, good-humored and other inward-generated aspects of “attractive”? If they are, then you should at least consider that your inamorata has so far demonstrated good judgment in her choices. So if you’re in her circle – at all – then you would seem to meet at leat the minimum standards for inclusion. (Depending on your assessment of the personalities of those exes, though… that may also be a low bar, or evidence of bad judgment on her part. And if guys with high standards reject her, or if she rejects guys who seem to have sterling qualities all around – then this may not be a good sign about her judgment or treatment of them. Keep these things in mind.)
You can’t change much about your physical attractiveness, but “accepting that” can go a long way toward a kind of psychic “comfort in who you are”. (You can work on fitness, and that is something that you should maintain and develop if you can – and it will help to improve and maintain your attractiveness, or what there is of it – for the rest of your life.) You can always work on the rest. (Maybe not intelligence, so much, but you can learn to emphasize the kinds of intelligence that you do have.)
You can always be better-read, interesting to talk to, friendly, personable, humorous, patient, forgiving, kind, thoughtful, hard-working, persistent (to a point) and self-deprecating (to a point) – y’dig? These are things that are only up to you. And to the degree that you can develop those qualities in yourself (and in others) you will be attractive.
Oh, and young. You should always be young, too.