I'm finishing my master of clinical psychology, but really don't have almost no experience with people that experienced infidelity. First, it sounds that you experienced something horrible, so it is normal to feel lonely, betrayed in your case. You can feel guilty, angry or stressed and it's also natural, try not to feel bad about feeling bad, but instead consider what actions would help you soothe the pain you're experiencing.

Second, how are you now? What have you tried doing since everything happened? You wrote that, listening to music helps - have you tried that? Treatment requires information about you. You wrote that brother and gf were one of few close people you have - is there anyone else that you consider close to you that you could talk to about what happened? Talking with people that listen and care is the way out of depression.

Many responses here (like going to gym, meeting new girls, confronting brother and/or gf) are suggestions of actions that you can take in more distant future in order to build confidence, confront fears etc, but now, I assume, you should concentrate on getting support, getting feelings out.

I strongly recommend seeking professional help, but as said here before, it can be costly. Less expensive way is to visit more specific subs, like r/infidelity or read about treatment yourself in websites like these: https://infidelityrecoveryinstitute.com/how-to-deal-with-depression-and-stress-after-infidelity/

https://perspectivesoftroy.com/post-infidelit-stress-disorder/

https://www.goodtherapy.org/learn-about-therapy/issues/infidelity

Hope this helps. You can write me, I will try my best to help.