It's important when people are bullying you online you seek support like you have which is a very bold move and shows that you are brave :) Well done. I hope the issue has been sorted and you took the advice given by the cybersmile team! If you haven't spoken to anyone yet I'd encourage you to. Another thing you could do is report the user on the game, i know on social networking sites you have options to block and report people. I'd encourage you to do that with this user as no one deserves to have those things said to them. I understand you being scared to tell your mum, but if it continues it would be good to let either your mum, a family member, teacher or another adult in your life know so they can also help to support you.
Hope your okay <3
My son uses Minecraft a lot and he gets the same thing sometimes. He usually has a number of mine craft servers with different types of players, some get aggressive and some just enjoy the game. You should maybe find a server that hosts nicer people, or can you turn chat off??