I don't really know the reason why, but I no longer want to be in my relationship.
Is there something specific about your relationship that you're thinking about?
I haven't been getting much attention from my boyfriend for a while now.
Relationships can end for many reasons and if you are not happy anymore it might be time to end things. Not feeling cared for and not receiving attention is a reasonable reason to breakup with someone.
What should I say? I've tried to tell him to chill out and that I want some space but he's just being weird about it.
You shouldn't be in a relationship that doesn't make you happy and you need to tell your boyfriend in a manner that is firm and assertive. It might be awkward, but so long as you're assertive your boyfriend will be okay and the two of you can end the relationship in a mature way.