"I asked for a break in January this year ... Anyway, we are on a break, still live together and are intimate. In my head, I suppose I just thought the break wasn't really a break because everything just carried on as normal."
I'm unclear as to what 'being on a break' means to you since you are still living together, having sex, and 'everything' as normal. It's clear that to him, being on a break meant he was a single man and could behave as such.
What were your expectation when you asked for a break in January?
Personally, I would be ending this relationship because of the original problem that "he kept looking at my phone and distrusting me based on nothing at all." As a general rule, when someone starts behaving in this way when you were giving him no reason to distrust you, it usually means that they are untrustworthy themselves. I think he was cheating on you back before January.
Cheats believe that everyone else cheats - it's how they live with themselves and their cheating. The logic seems to go that 'I'm cheating on her and she has no idea, I've no reason to think she's cheating but if I can hide my cheating from her then she could hide her cheating from me, and I'm cheating so she will be too and I'm going to go looking for proof'. It's a very twisted logic, and it's disheartening that it so often turns out that the jealous untrusting person turns out to be an untrustworthy cheater.
And of course, now he's blatantly cheating. I don't give a stuff if you were on this is-it-a-break-or-is-it-not, the fact remains that he chose to behave as if he were not in a relationship with you. He values his relationship with you (however on or off it was at that precise moment) less than the opportunity of a Tinder shag. And as I have said, I believe he was cheating on you before January.
So yes, staying in this relationship would be idiotic. Stop living together and give yourself the headspace to figure out what you want and where you're going. Give yourself a REAL break from him and you might find your thoughts have a chance to stop swirling and start making sense to you.