I care for my husband and his mother who are in different stages of dementia. Being with them 24/7 I begin to see myself acting like them.

All I can say is God love you. Caring for one person with Dementia is overwhelming in itself but two! I agree, time to place MIL.

Ever saw that movie 'Sophie's Choice'?

One of them has to go. Even if you bring in round-the-clock homecare help, one of them has to go.
Put the mother with late stage dementia in a nursing home. Now.
It does not make you a failure as a caregiver and is no reflection on your skills to be one.
Having been an advanced practice nurse yourself, you don't need anyone to tell you that in situations like you're in, the caregiver often dies before the care recipient. If that happens (God forbid) what happens to them?
Your husband is more important than your MIL. Put her in a nursing home.

You say in your profile that you have had no help to date. Well, that has to change soon. Today if possible. You must start bringing in some outside help, so you can get out of the house and do some fun things with friends and family. Doesn't have to be anything big, but it must be something. Going to church, lunch or supper with friends, going for a walk around the neighborhood, or just going shopping. Just giving yourself time away from the situation at home, and being around "normal" people, will help rejuvenate your soul, and help you to carry on with your caregiving journey. You must make yourself a priority too, not just your husband and MIL. I believe when you start doing that you will see a huge difference in yourself. Best wishes.

You need to find and make time for yourself. Find a caregiver so that you can get out for a few hours at least once a week. Meet a friend for lunch. Work in the yard. Do some things that you enjoy. You NEED to make time for just YOU.

Probably by not doing this alone much longer. I'm assuming MIL is farther along on her dementia journey, so I'd look into memory care placement for her and be prepared to have a place for your husband when he gets bad enough.

Do you have children who can help, or any paid assistance? You really need to be able to get out and interact with people who have functioning minds. If that's not possible, consider taking an online course to keep your own mind stimulated. Many libraries carry "The Great Courses" which have classes on everything from how to play the piano, to cooking, to physics and astronomy courses that'll make your head hurt. I really enjoy them myself, and you can do them at your own pace.