Don't want to have sex anymore?

I've been with my boyfriend one year

I don't want to have sex with him anymore, I still get horny but not with him. Sometimes we do oral but that's a far as it goes. I've even started to become uncomfortable with him touching and kissing my body, I get weirded out. I'm so in love with him, we're happy together but I know it's started to affect our relationship as i'm constantly rejecting him.

He doesn't put effort into himself anymore so he looks left attractive. I don't know what to do, I want to want to have sex with him, but I dont.

How can I want to? What is my problem? How do I tell him?