I have been sexually frustrated all my life and I have no way to relief this. I kind of have a girlfriend and like once a year (I'm not kidding) I find myself alone in the house (because either one of my parents leave and go to their empty house, they are divorced btw). So a few times me a this GF have had sex when he have our homes for ourselves, but it has been really bad sex. I feel all anxious because of the worry that I have to do it "secretly", also she has health issues (That we recently became aware of), and several other worries (like being stuck financially to my parents). I study performing arts, so financially its kind of gonna suck for a while, as I see no steady income soon in the future and there is no way I could pay for a rent for myself and my gf right now.

Another harsh truth is that I was never in very love with my GF, but I appreciated her and was mildly sexually attracted to her (She is also not very sexual). The last time we managed alone (like 7 month ago), she gave me a BJ, which it was more like a gesture of her to show me that she would like me to be satisfied. But what would really get me satisfied is to get sex everyday. I don't know what to do, I used to masturbate a lot. And as I used to masturbate almost daily, my girlfriend only touched herself like twice a year and to do a sort of "remote foreplay", yes, this unparallel our drives are. I don't want her to feel like she is not enough, but I am very frustrated. The idea of one night stand repulses me, but sometimes I see attractive women and I'd just like to have a quick relief and don't deal with romantic stuff and shit, but I am at the same time repulsed by promiscuity (so I have this moral conflict). I don't want to masturbate anymore (as I felt pathetic each time I did it). For me the answer would be that me and this GF could somehow share a rent together and sleep together more often (and maybe get married), but our family situations sucks (My mother doesn't like her, my career is slowly starting, etc..)...

I don't know what to do. Masturbation won't be an option anymore, it makes me feel like a total loser. I am planning maybe about leaving this girlfriend and start living the fuckboy lifestyle, but I am very shy and in the long term I really like this girl. 

I think you need to talk to her on how you feel . At your age  you will be probably at you most sexually active , so you will feel frustrated. If funds permit you could try some masturbation toys such as a stroker . This will give you a different experience and sensation  and you GF could use it on you as well.

Don't worry about looking at other women and fantasise about them,. You will be doing this long after your marry somebody. Your a red bloodied male right ? Women are not much different. Watch their reaction if a hunky fireman walks into a pub. Nearly all the women will look around. Its just the normal part of being human.