Didier,
I lost my father when I was young and all these years later I too can sometimes have waves of days when I dream about/think about him and feel sad about losing him.
One of my favourite aunts died suddenly when I was in my twenties ( about 20 years ago!) and that still unexpectedly comes to the fore and hurts sometimes as does the death of a friend about 15 years ago.
I guess I'm saying all this because I believe that firstly, as others have said, the loss of someone you truly love never completely goes away and, secondly, I think accepting in some way a heavy sad loss and getting on with your everyday emotional life takes as long as it takes. You may also naturally have some thoughts about your Mom and her cancer and those strong feelings may also be mixed in there with the sadness you are feeling.
It's only my opinion - hope I'm not saying too much - but I think you shouldn't be pushing yourself too hard to feel differently than you do- be kind to yourself. 18 months is not really that long.
Mary