I am SO sorry to hear of your situation. It must be truly truly awful to have to watch your mum suffer from terminal illness.
I lost my mum when I was 15, though the circumstances were somewhat different from yours.
It's not surprising that anger forms some part of your emotions at this time. Of course you feel angry and hard-done-by that you have to lose your mum when others can keep theirs for a few years longer.
These things are so illogical and irrational, there is no fairness in any part of it.
I'm sorry that your friends are finding it hard to empathise with you, particularly the one who lost her Dad. However, time is a great healer and maybe she has simply forgotten quite how much it hurt. Or maybe she lost her father in different circumstances and can't empathise with yours so much.
Friends are so important, now and also after your mum eventually passes away. You will need to rely on them and talk with them, as well as being distracted by them and forgetting your grief sometimes.
This is a time when you will learn who are your real friends and who are less so.
When you feel anger and need some space or someone to back off, try to avoid being snappy. Instead try to tell them calmly that you need some space. If necessary simply walk away.
But however you react or cope, don't worry. Your real friends will understand and will not judge you for it. They will still be there at the end of the tunnel when you need them.