my husband told me he loved me but that there was some part of him he didn't want to share with me anymore
I'm just going to say, this is unusual wording. How long have you been married? Has he had any affairs before? Does he have a history of depression?
I'm just asking because, well, the way he worded it is not the typical. The typical wording is "I love you but I'm not in love with you anymore". THAT wording usually means "I've met someone else that you don't know about and I'm getting my chuckles with her"
Now, it's possible that your H's wording means the same thing. Or that he's not cheating but just so depressed that he's drawing into himself. (Any triggers, like job loss or a death in the family?) But something about his wording also makes me wonder if he could be hiding something different. Could he be gay, or trans, or have run up a tremendous credit card debt that he doesn't want to admit to? Could he have an addiction?
I don't know why his wording makes me think of these alternate possibilities - certainly an affair is the most common situation. But something about the way he said it just makes me wonder.