Ummm... no offense to either of you, but you're both young. It's pretty natural to want to move on to other partners and discover different sides of yourself, but when you feel obligated to stay in a good relationship, lots of little cracks begin to show. Not having sex anymore than once every couple or few weeks is a huge, massive, gaping crack.
She says she loves it when you leave her messages saying "I love you" but never does the same, she's been wishy-washy about your relationship 4 times, and when you finally put your foot down- she didnt' have the guts to leave.
It sounds like you are both hurting for nothing.
I don't mean to be insensitive, but you're too young to do be doing all this soul-searchin in a relationship! If there's no pulse- it's probably dead. Move on. Neither of you are to blame. If you can manage to stay friends- so much the better. It sounds as if that is what you have turned into- really good friends with some lingering erotic feelings on your part.
I have personal experience where I stayed with a man for two years longer than I should have. I was unfulfilled in the relationship and so was he, but we both felt so obligated to try and make it work and stick it out, it took us a very long time for us to break up. The break up was amicable and we are still friends- but here is the difference between our case and yours... He is 34 and I am 28. We met when I was 21 and he was 28.
Btw- needing sex on a frequent basis doesn't make you selfish, a pervert or a bad person. Sex drives are natural and healthy and deserve to be expressed in respect and compassion. She is not respecting or being compassionate about your needs. Your sex drives are not compatible. Please move on before you drive yourself crazy. I wasted two years getting frantic because my needs weren't being met every two or three months- and that's how long he made me wait sometimes... it is so not worth it.
You are both young- you obviously need your space. Have a trial break-up and see how you feel. Go meet other people. If you (or she) feel excited and happy about your freedom, then you will know what is right for you.