To what extend should I keep doing useless stuff with my friends? Recently my behaviour changed a lot thanks to buddhism. 

I am no longer interested in watching movies, listening to music, and especially going out on parties and drinking alcohol.

This is creating a huge gap with my friends. My interests don't match theirs anymore. 
I tried to speak about the reasons why I feel better now that I left all these trivial activities behind, but I lack eloquence and thus didn't convince anyone. 

With my girlfriend things are easier because I can communicate a lot better with her, but I can still see that she suffers when I don't want to participate with the same activities as her. 

Should I accept to do trivial stuff from time to time just to stay close to my friends and not cause them suffering ? 

Following the noble eightfold path should make me easier to live with, but I just cannot watch TV one hour every night with my girlfriend anymore, nor can I spend my entire week-end going out and get wasted with my friends anymore. 
People around me find me boring and pressure me to do stuff I don't like anymore. 

On the other and I feel great compassion toward my friends and really wish to help them. 
If I stop seeing them, it would only cause more suffering. 
How can I adjust so that I can follow the path and not alienate myself by refusing to drink alcohol and watch tv ?
You are not causing suffering to others. They cause their own suffering if they want you to get drunk with them &amp; you refuse. You should always have metta (friendship) towards old friends but this does not mean you engage in unwholesome actions with them. 

Also get rid of this condescending attitude you have compassion for your friends and wish to help them. This is disgusting. Your friends are merely “ordinary people” who live ordinary worldly life.

As for your girlfriend, it is love &amp; respect for her that you watch TV with her. It is only TV. You don’t have to attach or delight in what you are watching yet this should not stop you showing warmth and companionship towards your girlfriend. 

I am typing this on an airplane because I am visiting my mother &amp; sister. I watch TV with them even though I do not watch TV at home.  Watching TV can be a meditation upon the ignorance of the world.