It's no wonder you don't trust him!
I don't think cheating means the story is doomed, over forever, etc, but in this case it's looking bad. He repeatedly lied to you, acted behind your back, etc. Poly is hard to take for a mono, so you need to know you can trust your partner even more. You need to be able to rely on them, on that safety of knowing where you are standing, how things are, etc.
If he lies and deceives you, you don't even have that to try and help you handle poly. That would be really, really hard, and yes, I can believe you have a bad taste in your mouth, anyone would after being treated like that.
It seems to me the problem here is with the people involved. I don't know how salvageable it is - maybe with a lot of talking you'll be able to rebuild trust and work from there. But it would take patience on both of your parts, and he'd need to be trustworthy, which he hasn't really proved himself capable of so far it seems :S
Me: 31F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon