He ... feels like we are house mates now
Why do you think he migh be saying this? Is there any truth in this from where you're sitting?
He's only said it's been getting worse this past year. He doesn't know why but it's got to the point he's felt he's needed to tell me but he didn't know if he wants to try to work through it or not.
I fear that, for whatever reason, he has already detached himself from you. He is planning his getaway. It is a truth universally acknowledged that men generally don't leave a comfortable setup unless they have already arranged a cosy nest elsewhere. In other words, chercher la femme...
He has been planning this for a year, so he is miles ahead of you. His loyalties are now elsewhere. Right now he may be feeling guilty, but this won't last long. So you need to catch up. Fast!
I totally understand how devastated you are right now. But your future self will thank you if you dont fall apart right now. Nail down your finances, so he cannot swan off with your joint savings. Take copies of all financial stuff: bank statements, investments, house deeds, mortgage, salary slips, P60s, pensions etc.
Get up to speed regarding divorce:
Wikivorce
Divorce for Dummies or similar
Consult a competent family solicitor
Even if it ultimately turns out that you do not need all the above (there's hoping...), remember that Knowledge is Power. If ever you need to be strong, this is it. If you were to be a pushover, he'll pity you and rob you all the same. If you're strong, he'll respect you and you'll thrive.