I'm sorry to hear that your mum has cancer. It sounds like you're experiencing a completely normal reaction, so you shouldn't worry that you don't know what to do, because this is how most people feel! When we find out that somebody close to us has cancer it comes as a real shock and people feel many emotions including sadness, anger and depression, and like you they describe feeling upset a lot of the time. The most important thing to remember is that you don't have to cope with all these emotions and this upset alone. We always advise people to try and find somebody they can talk to, so that they don't let their feelings bottle up inside. Have you tried talking to your mum about how you feel? This may seem difficult at first because maybe you don't want to upset her? However, if you can talk openly with her she will be able to help you get through this difficult time, and she will also benefit from having somebody to talk to as well. It may also help for you to have somebody else to talk to like another family member. They will also be worrying about your mum, and sometimes it can help to know that other people have similar concerns to us. I think friends are a really good source of support at a time like this as well. Although you may wonder if they can actually understand what you're going through, they will be important to help keep your spirits up as much as possible. If you can try and have some fun and make sure you look after yourself, you will be better able to look after your mum when you're at home.
Young people often comment that they don't know what to do when a parent has cancer. Adults try and protect young people because they don't want to upset them, and so this can leave you feeling like you're not quite sure what's going on. Sometimes it can also seem like you're not involved in what's going on, and you can end up feeling a bit helpless when all you want to do is help as much as possible! There are lots of things you could think about doing to help her out a bit. Maybe you could help her around the house a little, or pick her some flowers or treat her to a present. Most importantly, by just being there you will be a huge source of support and distraction for your mum.
I hope this helps, and remember that everybody deals with difficult situations in a different way, so you shouldn't be worried that you don't know how to act. Just try to find yourself somebody to talk to, and then you will be better able to cope with whatever happens in the future. Take care, and if you need any further support, get in touch as we're always here!