I just want to write a quick post to thank those who have chimed in with support and advice over the past few days - I feel very lucky. I chose to post instead of replying to comments because I wanted to make sure people saw this.
As for the advice to go into therapy, I think it's a great idea. We were seeing one for the latter half of last year but stopped because things were going so incredibly well. She's excellent, and a lesbian mother herself, so I assume she knows what this all entails. She already knows many of our (past?) issues, and I think it's best to use her to deal with this in the fairest way possible - for both parties.
We call it Thousand Dollar Therapy because of the cost... but some things are definitely worth the money.