the answer is simple. You are basing this entire relationship on her appearance and the sex. According to your first post there is nothing else. You two are not a match, you find shortcomings in her and your going to find the relationship lacking. Not only that , even if you say nothing she'll know that you find her lacking and it will make her feel bad. This is not based in age, but the fact that in a relationship the partnership should be equal and it is far too obvious that you are on higher footing then her.

The best thing to do is to break it off and tell her there is probably someone out there more suited to her, perhaps someone that is older then her but younger then you. Or you could just ignore the huge disparity between your lives and just fuck her as much as you can.