How do I tell him it’s not working?

Hi. I need advice! I’m dating my partner for almost 5 years now. We have our ups & downs but lately it’s getting worse. Today we talking marriage tomorrow he’s wondering what he’s doing with me. Honestly daily I also ask myself why I’m still with him. He’s selfish & spiteful. I’m not saying I’m perfect but he refuses to talk about things so I never know what’s wrong.

Due to the loneliness & uncertainty I’ve turned to my phone for comfort. I’d chat to my friends & sisters then he would get annoyed & accuse me of cheating. I’ve even started reading books & blogs just so I can stop thinking of my troubles. I met a guy last year, we exchanged numbers but never pursued anything. A few weeks ago I bumped into this guy again & we’ve been chatting. He says the right things & he’s sweet. Thing is I don’t have feelings for him & he wants to have a relationship. I enjoy his attention though but right now I feel I should rather break up with my boyfriend & focus on me. I have two questions :1. What are the right words to tell this guy I’m not into him? I don’t want to hurt his feelings. 2. How do I begin to tell the man I’ve been with for 5 years that it is not working & it’s best to part?