My 82 yr old MIL lives with us and it is really starting to affect our lives. Any advice?

She has been living with us for 4.5 years. She is so miserable all the time. She lives in the past and dwells on her past decisions about her life, her regrets and to top it off she was a substance abuse counselor for 30 years and understands the benefits of counseling but refuses to go. She physically has pain from her neck down due to arthritis but won't see a doctor or pain management about it. She had never been one to exercise so she has no strength. She is very stubborn and very religious. She has diabetes, and as a result her feet/legs don't always work the best, she has a cane but always forgets to use it. She is an odd bird in the sense that she has always slept sitting up on a couch, it is no wonder her neck kills her. Her eyesight is not good, and her hearing is not good either, yet she refuses to wear her 6k hearing aids. She has fallen 5 times since living with us, luckily, she has not broken any bones. When she first moved in with us, she basically took over the kitchen, it was her cookware, dishes, knives, etc. Her and my fiancé cooked a lot together. I relinquished my kitchen because they both enjoyed cooking together so much. About 6 months ago she was placed in the hospital due to a bowel obstruction, she had surgery and came home, her recovery was good and without any issues. She was more active before this happened, she loved to cook, she didn't mind doing dishes and little stuff but now, after the surgery she no longer cooks, she has no desire to even wash her own coffee cup. My fiancé and I make sure she has whatever she needs. She can't drive anymore, so he takes her to the store, doctor's appt. She occupies 1 side of our house, 3 full rooms in total. She is somewhat of a hoarder, she doesn't clean, and doesn't want anyone to clean her side of the house. She shows very little happiness, if at all. My fiancé and my relationship is dwindling, he is 59 and I am 58. We are existing not living, we can only leave her alone for a couple hours at a time for fear that she can fall. We have cameras in the house to monitor her while we are away...we haven't been on a vacation since she moved in, we use to go to the beach for 3 or 4 days but can't even do that anymore. My fiancé has his own health issues, he has his bad days, when he can't do it I step in, we help each other. Most recently she is displaying significant memory loss, she cannot remember somethings from 5 minutes ago. My fiancé and I have about 10 to 15 years left that we can do what we want to do, travel, and enjoy our retirement but it feels like we are on a very slippery slope navigating the care of his mother. We could hire in home care while we are gone but he feels she will be resentful to us and mean to them. She was mean to the nurses that came to our house for wound care after the surgery. I have mentioned assisted living and he feels that she will hate us. I am at a point that I don't know what to do but feel that time is speeding by at an alarming rate. I am not a decision maker in this scenario or don't feel that I am. I can't walk around naked in my own house if I wanted to, I can't play music. I stay too my room or office. I am contemplating walking away. We are engaged but have no date set. We have been going to couples counseling, but this topic has not come up yet I feel his mother comes first, and I am in a far second. I am at a loss... Help.