Not read the whole thread.
First of all
for OP. Horrible to have someone betray you.
The only qualification I have to post is that I have "form" for getting into relationships where the man cheats on me (in my case both very long term one marriage).
This is where you went wrong:
"He cheated on me a couple of years ago with a girl who he'd sworn was "just a friend""
In my experience the vast majority of men "reveal their colours" when (and if) they cheat the first time. The details of when they did it, how they did it and importantly how the fall out was resolved (if it was) says everything about what they will take forward in any future the relationship holds (if there is one).
In your case your boyfriend thought (this is very cynical so take it that way but better to hear it in its worst form)
"I have got away with a 2 year relationship (i.e. this woman is seriously into me, is seriously committed and really doesnt want to end the relationship, then i cheated with a "friend" and the woman I am seeing took me back. This means that I can potentially have a serious long term relationship with this woman whilst cheating on her on and off with someone who is a "friend"".
Men will try to get away with what they can get away with. What they can get away with is benchmarked by what the history of the relationship has told them to date. Your boyfriend thinks he can railroad you now even tho he is extremely likely to be seeing this other girl (whoever she maybe but she will likely be a "friend") at the same time as seeing you because that is what your relationship history has taught him. Men are that straightforward. They really are.
If you want a long term monogamous faithful to you alone relationship (you deserve this BTW if you do) then you need to dump this guy and fast. 5 years is a long time to devote to someone so low.