You remind me of myself, from 15 years ago. There is so much I would like to tell you.
1) If I could give you only one piece of advice, it would be to read the book Awareness, by Anthony de Mello. The book includes short talks on a variety of topics, but the discussion of relationships will shake up your perspective, which will probably be a growth experience.
2) It sounds like you and your girlfriend are a little un-confident. It sounds like you depend on each other for your sense of self-worth. When your self-worth depends on the opinion of another, you become a slave to that person. Slavery is not a healthy condition. If you and your girlfriend don't grow out of your mutual slavery, together, you'll eventually need to break up for your mutual well being.
3) Ugly girls are starved for attention, this causes them to behave strangely. Beautiful girls are overwhelmed with attention, this causes them to behave strangely. When you understand, in your heart, that beautiful women are as cursed and warped as ugly women, inner peace will come to your more easily. You will find it a lot easier to love all women for who they are. You'll feel less compelled to look at every pretty girl that walks by.
4) As you get older, your sex drive will probably mellow a little. Surprisingly, this is one of the best things about getting older! Instead of being mute and stupefied in the presence of women, you will find a broad range of women pleasantly intoxicating. You'll enjoy just chatting, or meeting for lunch, or smiling. You'll cease to care whether the woman in front of you has a boyfriend, because you're enjoying the simple pleasure of her company, in the moment.
5) You are probably over-estimating the men and women around you. As you learn to judge them more accurately, you will care less what they think. Focus on your long-term personal development. It's the only thing you can control, anyway. You'll eventually find yourself surpassing people you used to desire or envy.
6) Pretty girls do
like to be looked at, by desirable men. If you're not a desirable man yet, all you need to do is avoid leering and go about your business with a relaxed attitude. If you sense that an alarmingly beautiful woman may be in the vicinity, challenge yourself to glance only at her face. Leave the rest to your peripheral vision. Think of it as a personal development game. If you frequently find yourself standing around, glancing at attractive women who aren't interested in you, you're obviously wasting time.
Find a way to use that time for personal development, instead.
7) If you're worried about becoming a desirable man, don't take the opinions of women too seriously. Most women couldn't tell you how to be a man, even if they had the inclination to try. Bathe, groom, nurture your health, mind, and spirit. Keep working on your personal development. Eventually, you'll notice women who are "out of your league" starting to give you a second look. Suddenly, you'll realize that everything is going to be splendid!