You have contacted us with a couple of questions and I'll try to give you some advice on what to do to get the help you need.
It seems like you have a lot of responsibility in relation to your mum for someone who's only 13 years old. I don't know if your mum receives help from anyone else at the moment or whether she is in contact with a hospital or GP in relation to her cancer illness. I think it would be really good for you to get in touch with people locally that can help to support you emotionally but also advice you on how you can get some practical help in looking after your mum.
If your mum doesn't already have the right contacts, I suggest that you perhaps contact a Young Carers group near you to get some local support in relation to your caring responsibilities. You'll find details on local groups here. They should be able to help you in relation to the difficult situation you're in right now.
You also explain that you don't always know what to say when you're talking to your mum about cancer in particular. That is a very common way for people to feel and the best thing here is to be open and honest when you're talking to each other. Remember that there is no right or wrong way of talking about cancer and how you feel about it. Don't worry about getting upset and don't think that you have to put up a brave face to protect your mum so that she doesn't get upset. If you both pretend that you're doing okay, then it is much harder to talk about things that's worrying you and you'll both feel very lonely in addition to all the other emotions you may be dealing with. Just say what's on your mind so that you don't bottle everything up inside you. If difficult emotions are kept hidden inside us and we don't ventilate how we feel, it just grows bigger and gets more difficult to deal with as time goes on.
Hopefully, you'll get in touch with someone locally now so that you can get the help and support you need. If not, please get back to us again and we can talk things through or point you in direction of other local services.
Good luck with it all!