I dont know how to repent. Everytime i try i lose the guilt soon after and i think my repentance is not sincere so i repeat it.
And i have a big problem with waswas. Can you commit shirk in your heart. And do you know how i can get rid of these evil thoughts forever?
The first step of Tawba is feeling guilty about your sin. You must acknowledge the sin and feel ashamed of it and to stop the sin immediately and make resolve not to return to it. If you had harmed someone then you need to seek their forgiveness. This is sincere Tawba and have hope that Allah has forgiven you.
Remember, Allah forgives in abundance and very Merciful.
Discard all doubts as doubts are from Shaytan. Merely having Evil thoughts one is not committing sin unless it has been intended or uttered verbally.
To get rid of these thoughts recite أعوذ بالله من الشيطان الرجيم (A’uzu billa) in the first instance and making it a habit it will dispel it, إن شاء الله.You can read other Zikr too that you know like,
لا حول ولا قوة الا بالله، أستغفرالله etc.