Well, I can't promise you it's going to be fine -- no emotional relationship comes with that guarantee -- but it definitely does sound like you're overreacting inasmuch as you found exactly what you want and are freaking yourself out over unknowable stuff. Give it a go, this guy sounds great.
There are resources out there to help if you have questions or get jealous. Let me suggest the essays about being a secondary partner (limited time and limited blending of lives) at www.morethantwo.com
The major players. Me, 30ish bi female. Gia, girlfriend of 4+ years. Clay, boyfriend/dom. Davis, ex/friend/"it's complicated." Eddie, roommate & fwb.
The supporting cast. Eric, Gia's husband. Bee, Gia and Eric's toddler. Dexter, Gia's lover. Helen, Eric's lover. Izzy and Nikki, Clay's partners. Liam, Eddie's husband.