I don't know your entire story. I bet it's a long one.
Still, you gave off a bunch of points that tell me the following: your life, before you met the guy and got engaged, was shit. An actual cesspool.
And you got used to it.
Why am I suicidal again? I'm on meds...
It's the "again" that gave it to me.
So after five, ten, fifteen years of calling a hellhole home, suddenly, HAPPINESS!
This new and terrifying concept!
And you're yanked into it whether you like it or not!
Suddenly, there's this guy who legitimately likes you to the point of engagement, he sees your flaws as attributes, holy SHIT is this all a new thing!
Also, meds. You may want to talk to your doctor about switching meds. There are no perfect meds, there's just good-ish, bad, and shit.
"I'm like a dog chasing cars, I wouldn't know what to do if I caught one, you know, I just do…things" - Heath Ledger as The Joker
As cliche as it is to incorporate movie quotes in such things, I think it summarizes the situation well. If I suddenly mutate into Jimmy Hendrix, i'd lose my shit too. That's my dream. You seem to have achieved yours.