Help, please...

This guy and I were together for nearly 2 years... on and off but always fighting. And it got to a point where I just couldn't handle it anymore. I loved him to pieces and he knew that, but I couldn't deal with the constant fighting anymore. So I suggested that we take a break for a bit from the physical to try and end the fighting, but still remain close friends.
In this time he started to date my bestfriend and didn't tell me.. and neither did she.
I was so angry because I found out through other people, and these were supposedly my best friends!
I was so hurt and angry that I pushed them both away but the thin that hurt the most was that they didn't care and didn't even try to fight for my friendship, they were willing to just let it go.
Anyway, I finally started to get over myself and started talking to them again (things were never quite the same)
when another one of my bestfriends came to my house and told me that she'd slept with my ex whilst him and I were still together... and she felt so guilty and knew that he wouldn't tell me.
The anger started up again.. but the next day my bestfriend (the one who is currently going out with the ex) mums died.
I can't forgive him... I can't even look at him.
But I'm trying to be friends with the girl whos mum died... but she doesn't seem to want to accept it. She's so happy just being with my ex.
I don't know what to do.
I don't know whether to tell the ex that I know.
I don't know where to go from here or if I'm being really stupid by holding this grudge.
Please.. some advise would be really helpful. I'm dying here.

"I finally started to get over myself and started talking to them again"

That's your whole problem, she wasn't your friend and he was not good boyfriend material, cut them both loose and stop sabotaging yourself, pass on your condolences and move on with your life. (Pick better friends and relationships)

Like above, she wasnt a true friend to that in the first place, and well he obv wasnt a good bf either....just forget it all...looks like theydont show a sign of wanting to be friends again... and like you said...you were doing a whole lot better without them