Really need advice. My Mom (71) with dementia, can't walk?

I really need advice on what to do. She cries 24/7, she is on depression/aniexty medicine but the doctors say it's normal.. I can't feed her when she is upset or change her. I feel like I'm at a dead end and I want to give up..

I'm sorry you and your sister are being thrust from the nest at such an early age. I realize that most people your age are ready to strike out on their own, but for most it is nice to know there is a safe haven awaiting them if they need it. The reaction of her daughters speaks to why you went to live with your grandmother, they are both self centered and emotionally stunted, the very least they could have done is to help point you in the right direction.
Grandma need to be in a nursing home, she needs a level of care that is unrealistic for you to provide. She is only 71, I know that seems a great age to you but trust me, she could conceivably live 5, 10, 20 years this way. My mom was 90 when I moved in to help her, anticipating her death at any day. 5 years farther on she is still here, and I am stuck in a limbo land.
Gather what resources you can to understand the legal and financial steps you need to take to get grandma the care she needs and to get yourself launched in your own life. ((hugs))

lovesagoosa, in a previous post you were asking about Hospice for your Grandmother. Did you and your sister change your minds about calling Hospice?

https://www.agingcare.com/questions/18-year-old-taking-care-of-mom-191401.htm

lovesagoosa, you and your sister are so young. You're also so dear for doing this. I believe, though, that you need to find professional help to take care of your grandmother. Her needs are greater than you and your sister can manage alone. Please talk to her doctor about finding her a good place where she can get help. You can still visit her often. Right now are critical years in your own life. You need to be going to school, finding mates, building careers. If you lose these years through caregiving, it will be hard to get your life back on track.

There are so many options out there for your grandmother. Talk to her doctor. Call your local agency on aging (human resources department) to request a social worker to do a needs assessment. You are exceptional granddaughters, but this is totally unfair to you.