Oh dear- you have so much weight to carry on your young shoulders! I don't know or understand what your particular kind of cancer is- or how it affects you, but I do know that this stress isn't doing you any good!
Can you get your Doctor/Oncologist to write down the nature of your illness for your boss to see? He must believe it when it is coming from a professional person who is caring for you.
Can you talk to your Mum & let her be your rock to lean on? I know she has her financial worries but I'm sure she is concerned for your health & happiness. She may not realise how you really feel.
It must be hard with your husband away so much.... Does he get leave to come back & see you?
As for your friend, I think you have to explain to her that though you will always be her friend, you have an obligation to look after yourself first or you will be in no condition to help her. She needs some advice from a professional I would say.
It isn't right that are so unhappy at your age & I really hope things improve for you!