This was my first relationship and it lasted so long, part of me thinks I'm just throwing away something I put so much love and time into.

Never ever fall for the sunk cost fallacy. No matter how much time, energy etc. you've put into a relationship, if it gets unhealthy/abusive and/or doesn't make you happy anymore, you break it off.

I almost feel guilty for breaking up with him, and it confuses me because my emotions don't line up with what logically makes sense.

Give it time, you're still in shock about the whole thing. Everything's happened really quickly. Just don't make any rushed decisions now.

but because now that I took a step back it's so obvious that I was the only person he ever treated badly in his life. I feel like I'd be less upset if he just had anger issues and was rude/scary to everyone, but he's always really polite to his bosses and business partners, and he's the "nice one" in our friend groups

Are you really insecure of the behaviour of your jerk of an ex? Seriously, if he showed his true face to everyone, he'd not have anyone to talk to. Of course he's polite to everyone else who doesn't know him as closely.