Was I just resentful and unable to cope with all the change, was it the medicine, or a combination of both?

I'm sure whatever hell-hole city he made you live in more than justifies fucking another man behind your fiance's back.

How likely is it that the medicine really played a significant role in all of this?

Zero chance, sounds like the cheating, lying, all the way through the couples counseling, happened over months, do you really think it was a multi-month long manic episode? Sounds like you made a decision and stuck with it. At least until now for some reason, is the new guy not turning out so well? Then the real relationship advice would be to tell him before you do the same thing again to someone else.

Will it provide him with any closure (which I know he doesn't have yet)

And he never really will, that not something that happens after you do something like this.

do I share this information with Dylan?

Telling him you were on meds without letting him know, as you knew he would (rightfully is sounds like) think it was a bad idea, is not going to hurt anymore than the other lies.

To me this whole rant sounds like an excuse to explain yourself. Next month are you going to find out the stars were in the wrong position and get more relief? Try just accepting it, blaming the meds, in even the smallest way, is just another excuse and will keep you from coming to terms with what you are.