Please help with fears of death and infinty ?

I was born into a Christian family, but they weren't that strict. I went to a private school, and it wasn't until high school that I started thinking that God and an afterlife may not be real, and there might be nothing waiting for us after death. The past few years, I have had anxiety attacks for short times about re idea of not existing for eternity, but about a month ago I had a similar attack but of a higher magnitude. Thought it would just pass when o woke up like usuall, but now I can't get the thought out of my head. It has become an obsessive though that eats away at me and stresses me out. Right now it is the summer and during the day I am alone with my thoughts. I can't figure out if it is death or the idea of infinity that scare me more. Im only 16, but these ideas are really freaking me out. I guess what I'm looking for is something I can believe in that happens after we die.