It will hurt us both, no doubt.
I don't know your partner, so you might be right about this. But, other people don't necessarily feel the way we think they do. For a start, it sounds like neither of you is totally happy with the relationship. You having the courage to face that gives both of you the chance to move on and find something better.
My ex was extravagantly grief stricken, at first. Said he "didn't want to live without me" etc. (He also said he wouldn't let me go and then that he'd stalk me.... :rolleyes:) A week later, it was all my fault and he was spreading crazy stories to explain the end of the marriage. About a year later, he was remarried. What I came to understand with HIM (not saying it's this way with YOUR partner), but what I came to understand with him was that the whole relationship was about him all along. HE was grief stricken be HE wasn't getting things his way, for pretty much the first time in the relationship. People like that bounce back pretty fast because they don't experience any guilt and the "grief" they feel is mostly about not getting their way.
Actually, now that I think about it, the longest term relationship I've been in since the marriage was one I ended too. Long story & I'll try to do the condensed version. We worked at the same ranch, which went out of business so we all need to relocate. "We" came up with a plan, that he was more enthused about than I was, but I was ok with it. When the day came to move, he couldn't make up his mind and decided to back out. And then not back out...... And then we moved and he was in & out of here every couple weeks for several months. I didn't want to leave him with no place to go and I didn't want to be the bad guy and be the one to end things. But he was clearly not ok with moving 1000 miles from the place he grew up. Finally, he asked if he could come back and I said "No". Told him he was driving me nuts, needed to make a decision and get on with his life. He could live anywhere he wanted, but he couldn't live with me. If he wanted to move back HERE, he was going to have to find his own place, at least until he knew what he wanted. He stayed where he was and was married to someone else in less than a year.
Again, I'm not making predictions, just saying it's easy to assume that other people feel and respond like we do and that's not always true.
Oh......... Because we had a farm and I didn't trust him to take care of the livestock and it took awhile to sort out who got what, my ex & I lived in the same house for 5 months. Obviously we both survived, but I wouldn't recommend it if you can avoid it. I know other people who've had similar situations too. It helps if you have different schedules and a big house!