How long ago did your partner make this "announcement"? 18 years is a long time to just throw away.... anything that you can think of to have prompted it?
I know if my husband (coincidentally of 18 years) made such an announcement, I would be a blubbering mess. So, I'll give you the advice that I would give myself. (I can't say I would follow my own advice, but I hope that I would.)
Go take a walk. Just around the block. Sunshine is an excellent mood enhancer. Once you've taken that walk, open your blinds and curtains, open the windows if the weather is nice. Let mother nature soothe you a bit. Then, find a hobby. NOT TV. Puzzle, or knitting, or anything really. And if it's portable - BONUS! Take that hobby to your local park and sit in the sunshine while you play with your hobby.
And, please try to have a nice long talk with your partner when you're able. Suggest counseling or working together in some other way to improve the relationship. It's never too late. Treat your partner the way you want to be treated - even if that's not how you're being treated right now. Keep making that person a priority. Hopefully, they will come around after a bit.