It occurs to me that you are worrying about something that might not even happen. I don't know if you are "back together" in your relationship but you talked about much you love both tommyc and Sea and how the relationship as the (V? triad?) is what you want. You are feeling guilty and scared that someday tommyc might want another partner and you wouldn't couldn't stand it.
It might seem short-sighted, but what about living for today? I am all for living prudently and trying to make some rough plans to circumvent disaster, but I'm not going to cancel my trip to SE Asia because I anticipate tsunamis, malaria and muggings. They are very real dangers, but then again, they might not happen at all!
There was a song/graduation speech called "Wear Sunscreen" or something like that where this guy was giving the advice to graduates speech. In it he said "Don't worry about the future. Or worry, but know that worrying is about as effective as trying to solve an algebra equation by chewing bubblegum."
Often he had been often bound with fetters and chains, and the chains had been rent asunder by him, and the fetters broken in pieces: neither could any man tame him...and he asked him, What is thy name? And he answered, saying, My name is Legion: for we are many.