Rory, are you actually unhappy/dissatisfied with your relationship with Alec on your own behalf? Or would you be comfortable with the relationship if he wasn't feeling lonely? I sense the possibility that you might decide to get divorced because he is unhappy, and worry that maybe you're taking on a decision burden that more rightly belongs to him.
On a related note (and inspired by all the talk here lately of relationship anarchy) is it possible that you have a particular image of what marriage is supposed to look like, and that some of your discomfort might be alleviated by relaxing the constraints in that image?
Sounds like the last few days you've given yourself freedom to accept and explore how you feel, and explore different shapes your relationship could take. It sounds like a very positive step. And glad for you that you're able to share that with him, rather than feeling a need to hide it. All those "things I can't talk about", whether the relationship or otherwise, can come to feel like a huge barrier.
Good luck with sorting it all out!