I had idiots with two living parents coming out with mindless shit like this after mine died. I am sorry you've been on the receiving end of it.
There is a very weird and dysfunctional belief in our culture (it's not universal, it is specific to our culture) that after someone dies you are supposed to live as if they never existed, extract them from your mind, feel no connection or emotions, never think or speak or talk about them.
But that's not normal for humans. And I don't think it's healthy.
You still have a connection with someone after they die. They've shaped or influenced how you live and how you experience the world. You still have memories that stir emotions. You can still feel the closeness you had, albeit tempered by their physical absence and pain of their loss. You notice their absence. You have things you wish you could share or ask. You still remember their advice and the things they taught you. The times they let you down and the times you let them down. The love you shared.
It has been over a decade now since my mum died. I don't notice specifically how often I think about her now, but that's more because a) it doesn't cause such overwhelming bone-crushing pain to think of her now so it doesn't stand out in the way it used to, and b) she flits in and out of my thoughts so often as I go about my life - in the same way that my loved ones who are still alive will pass through my thoughts - that it is unremarkable. She's just still there along for the ride with me. It is certainly not the case that she has disappeared from my thoughts or that I no longer care.
For me it is still a wound and sometimes it feels unbearable, although mostly it is bearable albeit sore. I still cry and miss her very deeply sometimes though. Why wouldn't I?! That's normal.
I sometimes describe the loss as something I am able to carry now, but I also mean that I carry my mum with me through life, in a way that is healthy and manageable for me.
In my view, 9 months is still very soon and very raw, especially after the loss of someone so significant. Things are always different after losing your mum, it doesn't one day reset. People tend not to get that unless they've experienced it. I also think it frightens some people to be confronted with the impact of a loss that they will one day experience and don't want to think about.
That's a lot of waffle. I suppose all I am trying to say is that you're perfectly normal and you're not alone. It is completely ok if you still think about your mom on a regular basis for the rest of your life. 