How do I deal with the stress of caregiving for a mother-in-law who has become more difficult as she gets older?

MC/Nursing Home time

Being unable to toilet herself should have been your line in the sand. It will only get worse from here.

This is the tip of the iceberg, unfortunately. As the dementia progresses, the wandering starts. The Sundowning where the agitation ramps up, demanding to see her dead parents and siblings, insisting on calling them on the phone, hallucinating, making up stories (confabulating) about people coming into the house to steal from her or spy on her............all sorts of histrionics you can't even imagine are likely in store for you. Like when she refuses to bathe for months on end, then what? I had to move my mother from regular Assisted Living into Memory Care 3 years before things got very bad, and when she went into a wheelchair and needed a lot of help 24/7. As her dementia advanced, she required Ativan just to function because her agitation was through the roof. This is typical and common behavior for elders with dementia and if you have no experience dealing with it, the stress is unbearable. Start looking into Memory Care Assisted Living NOW, before things get to that point, that's my suggestion. And if your husband isn't on board with placing her, then have HIM do all the caregiving for her. Or have MIL move in with the other adult children 'who work full time'........who cares? You've done your part, now it's their turn. I'd go out and get a full time job in a New York minute before I'd agree to care for an 88 year old incontinent woman with dementia, that's for sure!

I suggest you read this 33 page booklet (which is a free download) which has THE best information ever about managing dementia and what to expect with an elder who's been diagnosed with it. I think your husband should read it too, so both of you know what lies ahead.

Understanding the Dementia Experience, by Jennifer Ghent-Fuller 
https://www.smashwords.com/books/view/210580

Jennifer is a nurse who worked for many years as an educator and counsellor for people with dementia and their families, as well as others in caring roles. She addresses the emotional and grief issues in the contexts in which they arise for families living with dementia.

The full copy of her book is available here:
https://www.amazon.com/Thoughtful-Dementia-Care-Understanding-Experience/dp/B09WN439CC/ref=sr_1_2?crid=2E7WWE9X5UFXR&keywords=jennifer+ghent+fuller+books&qid=1657468364&sprefix=jennifer+ghent%2Caps%2C631&sr=8-2

Good luck.

I would hire an aide for a few hours a week a few days a week. She pays. You get a much needed break.

How long has she been with you? Does your husband help on nights and weekends?

My mom lives with us and I have an aide come in 4 days a week for 4 hours. Extra aide if hubby and I are going out for a long time. It's given me back a small part of my life.

Good luck.

You start looking for the appropriate facility, where she will be cared for by trained personnel, be around people her own age and you can get your life back. And if she doesn't have the money, then she must apply for Medicaid.
Either that or your husband needs to quit his job to look after her as she's his mother.
You already know that she will only continue to get worse so it's best now to get the ball rolling for placement, before you end up being in the statistics where the caregiver dies before the one being cared for.

You should have a caregiver come in to help out. MIL pays for said caregiver.
Taking care of someone with dementia does not get easier. As she declines she will need more and more help. She will go from the quad cane to a walker then a wheelchair. Is your home equipped to handle her with a walker then wheelchair? If you plan on keeping MIL with you making plans to renovate is a good idea.
Or decide now at what point will you no longer be able to care for her safely and have to look for Memory Care for her.
Who is responsible for her? Does your husband have POA for finances and for health decisions? If not does anyone? If not there is the possibility that he may have to obtain Guardianship. You might want to have him schedule some time with an Elder Care Attorney.