I think my parents are going to ruin my birthday
What do you mean when you say they will ruin it?
It's the 8th birthday and it's like they forget that
It's okay to feel upset about the idea of your parents forgetting your birthday. Sometimes parents can be busy or tired from trying to get everything done. Try to be more aware of the things they do for you and gently remind them of special occasions in case they forget.
I'm going to hang myself with a belt, so they will remember.
I get that you are upset about it, and I think that it is okay to be upset that they forgot. I think that there are safer ways to tell them that you are upset though. Maybe you should just try talking to them instead.