Just that, really.
My partner lied to me about something major (he slept with someone else at the start of our relationship, when challenged he promised that he hadn't, I found out from someone else and he then confirmed it).
I don't know if I could ever trust him again. The cheating is one thing, and is bad enough. But the lying, repeatedly, is quite another, and in my eyes is much worse.
Can trust ever truly be rebuilt? I think a small element of doubt would be like death by a thousand cuts. But I'm interested to hear if it's possible to move on without the doubt always being there.