Is swearing an oath saying "if I do such and such, I will become a kafir" valid?

I was alone and then I got a thought that if I do this sin (masturbation) then that means I don't believe in Allah and then I don't know why I just gave in to the temptation and started masturbating. Does that make me kaffir? I still pray and do dikr and worship Allah but I get these kind of thoughts that if I didn't do this and that, then I'm a kaffir and I don't know what to do.