It is undoubtedly the right of your mother that you should be kind and benevolent to her and reach out to her when she is in need, but you also have to fulfill the rights of your husband who wants you to be in his presence. If you take this lightly, you will end up with misunderstandings between you and your spouse and God forbid it could dissolve your marriage. This would hurt your mother more than the situation she is facing right now.
Try talking to your siblings or your relatives who can manage to take care of her even though you know they might not do it whole heartedly like yourself.
If you can explain your mother lovingly about yourself and the rights of your husband who is calling you, I'm sure she will come to realise your situation and will definitely want your marriage to work out properly. You can always be in touch with her and make sure you do so that she can be satisfied.
Talk to her on line or anyhow possible to make her understand that you care as much even if you're far away.
With time she will realize and get used to this fact that you need to be with your husband also. Try to visit her again of you get a chance and your husband gives you the permission without forcing him.
I pray that you earn double reward for keeping the rights of your parents and your spouse inshAllah.