You seem to have a good head on your shoulders. You're not opposed to returning to monogamy if it's proven that poly just won't work for you, so that's a bonus.
It also sounds like you had a lot going on the last time you tried poly, and that always makes it harder. I always think you should be on a solid foundation before you start building additions. From your description, your foundation needed work, and you've worked on it, and now it's solid.
You're right to be suspicious of the idea that you can have just-sex with others and not allow emotions to bleed in. Some people can pull that off, but if you have doubts, then you're probably not one of them. You don't seem interested in casual sex with acquaintances, so that's not likely to give you the "vigor" you're seeking anyway.
My only advice is that if you go forward, make sure your partner is prepared for you to have romantic relationships with other people, because my hunch is that it will go there anyway, and if she's not prepared then she'll feel like the rug's been pulled out from under her feet.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker