I lost my dad when I was quite young (18), similarly to your situation we were very close and I adored him. He was the one kind and caring character in my family and when he died I was numb about it for a long time.
I used to dream about him a lot and always wake up stressed and upset because in this 'sleep life' I would discover that he hadn't died at all, but had left us and was living another life somewhere else - sometimes with another family, wife and children, sometimes on his own. He was always aloof and unwelcoming in these dreams and I would constantly wonder what I'd done that was so awful he would leave me.
I interpret these dreams as partly to do with my insecurities, partly to do with trust issues within the rest of my bog awful family but mostly I think it's my brain still trying to make sense of the loss and to find a 'reason' for it. It's very taunting, suggesting that if I can find him I can reclaim him somehow.
I wonder in your case whether it's your psyche telling you to finally let go and move on and to live with it, not through it. By manifesting your lovely mum as this horror your mind is trying to create a reason why she 'left' so that you can separate from her and accommodate the loss.
Sometimes we do get stuck when such a close relationship is taken from us and it doesn't matter how much counselling, or good advice we receive, it just doesn't change - mainly because at some level we are refusing to feel the feelings because it feels like they will overwhelm us and swallow us whole. They wont, but it is terrifying to think about.
Sometimes counselling can seem good way of dealing with something without having to feel the feelings - we talk about them, give them to therapist and in some ways hope they will be able to interpret them, wrap them in panic package, stick a bow on it and make the whole thing more 'palatable'.
Nope, I'm afraid you have to feel them - red eyed, screaming, howling, on the floor in a pool of tears and snot. Therein lies the strength to truly grieve, separate yourself from the person who's gone and move on with them metaphorically by your side, rather than trapped inside you and fighting with your mind to get out.
I hope all of that makes sense and that you can get to a place where 'conjuring' your mum is a warm and comforting thing.
I still dream about my dad from time to time - he's still not where I want him to be, but it's better, and he's pleased to see me now.
best wishes - it's never easy to miss people. x