<human>: I am M17, been with my girlfriend (F17 currently) for more than two years, still going strong. It's the in between that is the problem.
My girlfriend and I are pretty normal actually. We meet, we do simple stuff, go for meals as much as we can. Thing is, I don't really feel happy with her. I just feel 'not sad'.
I have met two other girls that have really made me feel happy and all, but I have put them in my sister-zone. Now both are happy with another and well, nothing much changes.
Here's the complicated part. My girlfriend and I had a break for about a month, and during that time, I had a small fling with one of the girls that truly made me happy. Even after patching back with my girlfriend, I still feel this huge sense of loss. Seeing this other girl, walking under her block, and seeing her with another guy really leaves an unpleasant sinking feeling in my gut. I don't only think of 'What if's, I think of 'I wish's too.
Now, I admit it's my own fault and stupidity that I landed myself in this state, but I am really trying to fix it. I have stopped demanding and expecting from my girlfriend, I aim to make her simply happy. I don't want to hurt her anymore. But if this goes on, it won't be good.
Seeking advice here, on how I can get over this other girl, and how I can cope with this relationship dynamic of giving all I can.
I know I should be appreciative of her and not expect anything in return for what I do. That is unconditional love isn't it? But sometimes giving one-sidedly can be tiring, and not being able to really feel happy can be well, depressing. So yeah, hoping for help. Thanks.

Summarize the given text in a few sentences.
<bot>: Not sad with girlfriend, but not happy either. Can't get over another girl I was really happy with, need help.