sex with my girlfriend is not so good

Sorry, but if you do not think she is attractive anymore then you must break up with her. It's bad for her and bad for you.
Fapping is definitely not helping you, and it's not helping her either. I'm sure she will be fine without you. And you need time to grow.

 

I lost attraction to my first girlfriend probably because of porn. I just went to her place one day and looked at her, and felt nothing. So I decided to end the relationship, there was nothing else I could do. Looking back I realize she was stunningly beautiful, but I could not appreciate it at the time. This is how porn damages the way you view other people.

 

Sexual compatibility is pretty important. If you consider that sexual novelty will only decrease in time you might end up in a deadbedroom which will most likely trigger PMO behaviour

 

You are not in love with this person, and you are wasting her time. You obviously like her, you are friends with her and you appreciate her personality, but that's not the same as love. She is not with you because she needs a brother or a friend; she is (from your description) a flesh and blood woman with flesh and blood needs, a passionate being with wants, dreams and desires that you can't fulfil. So let her have a chance to blossom by letting her go.

 

Could you imagine how it would be if you knew she compared your penis to other guys when you have sex ("he is too small")? If she imagine other guys who excited her more in order to orgasm? Would you be 100% comfortable with that? If she said that she "loved" you, would you believe her?

Bet not. She might be real and magnific, but thats not enough for you, since you are completely fake in your feelings for her. It's time to get real, man.