I totally agree with everyone else on here who have said that it is entirely normal and natural to shed tears and feel sad when recalling the death, in detail, of your Mum. Who wouldn't shed tears doing that ?
However, you have run up against a Social Worker, or a Supervisor of a Social Worker , who is just plain caught up in their own little world of 'working things through' (And probably one who has not suffered a major bereavement like losing a Mum). As they have the upper hand in that they can say yea or nay to your being allowed to foster I think you should follow ummm's advice and go to see a counsellor, who will say that you do not need counselling. Sometimes you just have to jump through the hoops.
Please try not to upset yourself further about this. Treat it like taking an exam in school. The Social Worker who dealt with you sounds to have the sensitivity of a custard cream.
I also agree with you that relating a sad memory totally dry-eyed , no matter how long in the past, would be a pretty good indication to me of someone I would not want to look after a child.
Do not let this idiot grind you down. Good Luck!