Maybe your mum has not come to realise there is no time limit on grief, everyone is different. What is 4 months!
Perhaps if you plan things so you see your 2 friends away from their normal crowd it would seem better, however better yet just be you and wait for events to change themselves.
You are still the same person with just as much to offer another as you did before. It just takes someone to see that rather than being preoccupied with themselves
You did say "I am a naturally friendly person but friends don't stick", I think you are making a basic mistake (sorry about that :)
There is a huge difference between true friends and acquaintances. Most in life you meet are in fact acquaintances. Maybe necessary for company at times, pleasant to be with, but no depth of feeling or commitment. Friends are another matter, reliable and caring. They are very hard to find. You can't look for them, if you are lucky a few will appear in your life.
You are a person that takes relationships seriously -values them. That is a wonderful way to be (even if it does lead to hurt at times) and your friendship once given would mean something.
It's tempting to stay in bed, maybe your mum is doing you a favor, in bed the world will not change and that's a depressing thought.
I hope we talk some more