Mom is forgetting to go to bathroom and change the depends. I feel bad to keep telling her. She doesnt understand. How do I talk to her?

You can't "talk" to her. She has Dementia. You are now going to have to be pro-active. Take her to the bathroom every couple of hours. Try to figure out when she has a BM. You are going to have to help her change her Depend and probably clean up.

Your profile says mom is still in her home. Maybe she isn’t able to be alone anymore and you should start looking for facilities for her. It gets harder to manage on our own as we age.

I wish you and your mom all the best.

If she could do better, she would. I'd take it that she can't and get her help. Does she have someone with her at all times to accompany her to the bathroom each time? That might help let you know what's going on. She likely doesn't have the ability to do it. My LO wanted to keep the same clothes on, partly because, she wasn't able to coordinate pulling shirt over her head and getting her arms out of shirt.

I'd keep in mind that your mom likely doesn't have the same sensibilities about things like toileting, as she did before. My SIL changed her father, who had dementia for many years. I'm not able to do that. Some people aren't. I'd consider that aspect and explore who will do it. At this point, some people consider placement or bringing in outside, full time help.

This is the stage where you step in because she needs more help.
She is probably needing more help showering as well. It might be a good time to do sort of an ADL (Activities of Daily Living) evaluation and see what she can actually do for herself and what she is needing help with.
If she is unsteady on the toilet maybe a walker turned around and backed up over the toilet will provide stability when she sits. Is the toilet high enough that she can sit and stand easily? If she needs grab bars the reversed walker will help but they are not anchored and you may need that. Are there grab bars in the shower? Is it a shower she can get into or does she have to step over a tub? Is there a bench or seat so she can sit while showering?
You will have to go intot he bathroom with her to determine a lot of these things.