I'm sorry that you're feeling this way & I'm also sorry your Mum isn't happy.
It's important to you that you support your mum, and that is really special. However it is also important for you to support yourself.
It is not your sole responsibility to look after a parent. Does your dad know about what's going on? I think the first step is to lessen the responsibility and get other people involved that your mum trusts and who love her.
Perhaps you could begin expressing some of these feelings to her that you love her and want to spend as much time as possible with her, but you also have things you need to get on with without her? Maybe you could recommend her speaking to a therapist?
This is largely about what makes you the most comfortable. So if my suggestions make you feel uneasy or apprehensive then of course find something that sits right with you.
But I think you need to find a way to take the pressure off yourself but still ensure your mum has enough support, that just doesn't have to be entirely you.
Let me know your thoughts?
All the best,