How do I get over the feelings of guilt in the dealings with my 89 mom?

This is really difficult since she is already living with you and the disease will continue to get worse and her needs more demanding. Start opening up conversations with her now.

Make sure you have all the legal paperwork up to date including DPOA for medical and financial.
Next, have the hard discussion about how she wishes and where she wishes to live or have care going forward. That time may come when you and your family no longer can meet her needs.
Understand her finances and research resources you can tap into to assist with her care needs.
Start getting some outside help in the home -- cleaning person, companion for mom a few hrs a week, etc so she can depend on someone else as well as you and get used to having someone else attend her needs.
Encourage her to go to senior center for activities while she is still able. Visit and consider adult daycare programs in your area to give you a break. They can be very reasonable.
Take care of yourself. Make time to get out with friends or have friends over so you don't isolate yourself.
Consider attending caregiver meetings that might meet at your church, community center, hospital, or AL, dementia care facilities.

Continue to visit this forum and gain additional insights and yours and moms future. Many have walked your path and have a wealth of knowledge and empathy.

As to the feelings of guilt -- they may increase along with the dementia. There may be conflicts ahead as you arrange for more help, have to set limits for Mom's safety, etc. She may not understand and like what you need to do. I think that guilt seems to go with the territory with caregiving, and especially caregiving involving dementia. We want to do our best. We strive for perfection. But in dementia there is no perfect solution, so we always fall short of our own intention.

It will help to learn all you can about your Mother's condition. Set your goals realistically. Aim for "good enough" instead of perfection. Let go of unearned guilt as much as you can. You did not give your mother dementia, and that is what is at the root of her problems. Not Your Fault.

Some guilt feeling may still hang around. Push them to the background and make the best decisions you can for your mom, yourself, and your family. Don't let guilt control your decisions.

sunflo2 has given you good advice about planning ahead and about being sure you have some respite.

Best wishes to you as you embark on this challenging part of the journey.