I'm so sorry you are going through this, especially at such a young age. You are so brave to be facing this though. You are spending your time with your dad, it's really all you can do. It's the worst thing to go through, there is no easy way to explain that to other people who aren't going through it. That's probably why you don't want to see your friends. You are in a completely different place in life right now that many people won't understand. However, you would be surprised at how kind and empathic people can be. If you find that friends try to reach out to you, you might find that you are more wiling to see them after all. Take all the love and support you can get right now. It's so hard to go through it all alone, at any age. 

For me, the hardest part was when my mom was at the end. My world fell apart and it was just all about being in the hospital with her. There's really nothing else. However, it is also important to find a little bit of time for yourself. Try to take a walk outside, or maybe watch a movie. For me, these were ways that I could cope with it all. 

Just keep staying strong, don't be hard on yourself or question whether it's normal. It's incredibly difficult stuff you are dealing with, and I really am sorry. 

Hello michou123; how awful for you to have to lose you dad.  There is an website for teenagers who have a parent with cancer called riprap and I am attaching a link in case you would like to take a look.  Sadly you are not the only teenager who is going through this and it may help to discuss with others in your situation.  I am so pleased you posted here; this is so difficult for anyone especially when you should have had so much more time with your dad.  Annie

http://www.riprap.org.uk/