Of course you feel conflicted--you've had a long relationship with him. But the problems you mentioned are important ones. You've said that you have tried multiple times to fix things and it hasn't worked. So it looks like you've treated him with respect and done everything you can do.
The possibilities you mentionedof ending up in a bad situation with another manare the reason why so many people stay in unhappy relationships. The thing is, you KNOW you'd be unhappy if you stayed in your current situation. That's a guarantee. So you have a better chance of finding happiness elsewhere than you do by staying where you are.
You're used to your current partner's issues and might find that it takes a while to figure out other people's. In other words, don't rush into anything. I have dual citizenship and one thing I find very useful about the US is the concept of "dating." It doesn't seem to be popular in the UK. Dating means getting to know someone before acknowledging a deeper emotional connection. It's a way of protecting ourselves from getting attached to someone and then realizing things won't work out, but feeling tethered either by emotion or circumstances (living together, etc).
I'd add to that that it might help you feel more in control of your fears if you sat down and figured out some boundaries for yourself. For example, what's a dealbreaker for you (something that immediately puts you off wanting to be with someone)? Maybe it's being yelled at, or going a month without sex, or financial irresponsibility. If you go into your future encounters with a clear idea of what you won't put up with, you may feel more in control and waste less time agonizing over whether to continue seeing someone iffy.
Honestly, I think you're doing your partner a favor. I hope for his sake you've been honest with him about why you're leaving. Maybe he'll eventually get that his unwillingness to participate in your relationship drove you away.
You may indeed end up in a bad roommate situation or dating weirdoes. The thing is, you won't be trapped. Your actions hereditching a relationship that is not providing the basicsshow that you are sensible and make good decisions, so I wouldn't worry that you'll suddenly lose the ability to get out of bad situations. Also, a few weird experiences make life interesting. And every time you have one, remind yourself that we have to put ourselves out there in order to encounter good things/people.
You'll be fine. My advice for the immediate future is to join meetup or the gym or borrowmydoggy.com and start filling your time so that you won't be obsessing over anything once you're no longer living with him. Good luck.