These things take time, Andrew. It can feel very isolating when others seem to bemoving on with their lives, and you find it so difficult. I dont necessarily think it's because they don't care, it's more that certain bonds we form are so much harder to bear when they've gone, but that's the price we always pay for that precious relationship.
I know that although it's been very painful losing our treasured companions over the years, I wouldnt trade the relationship I enjoyed with them, to save myself from the pain afterwards.
I think that when you feel more ready, taking on a kitten or a young cat might be a good move. It's hard not to get enthusiastic about their energy and zest for life, and your other cat (what's her name by the way?) may well welcome the company - and if she doesn't, then you'll have to work very hard to get them to trust each other, which will engage all of your senses again.
I know it may sound like a gamble, but often the thing which helps us on our own road to recovery is having something else to focus on which absolutely relies on us to get it right, or at least give our absolute best in trying to.
I think the move with regard to volunteering is a positive one, for similar reasons.
Don't discount going to see your GP and asking about bereavement counselling. Your health and wellbeing is a precious thing too, and sometimes it needs more help than uual to get back on track.