Sorry to hear about your mum
. My mum died getting on for a decade ago, and it is such a loss. She was lovely, and interesting, and endlessly fascinating to talk to. She lived long enough to see DS, but I really miss the chance to talk to her about motherhood. But one thing I remember in the aftermath of her death was a feeling (self-imposed - no one made me feel this way) that my grief had to take second place to my dad's because he'd lost his spouse of 46 years. I wonder if you could be feeling the same?
But one thing I really treasure the memory of how sane and sensible her advice could be. In particular (and relevant to your situation) I remember my mum talking to a college friend of mine about this - he was trying to come to terms with his father meeting a new partner a year after his wife had died (it had been a very happy marriage). My mum explained that these days it is relatively rare to be widowed (compared to how it was in her childhood - she lost her mother to an infectious disease which would be readily treatable with antibiotics these days), so our model for the end of relationships was break-up rather than death. And break-ups are traumatic and take a long time to heal from (she divorced her first husband, re-married 5 years later, my dad sometimes used to tease her gently that such was her fear of marriage from first time round that she'd looked more like a woman going to the dentist than going to be married). But with a happy marriage, being widowed is utterly hideous, but it doesn't leave you feeling that the whole business of partnership is tainted and dangerous - quite the reverse in fact. So in her childhood it was not uncommon for people who had been happy in their marriage to meet someone new and remarry relatively quickly.
Since then, I've seen this happen with a handful of friends, and to me it honestly does not seem disrespectful or wrong. However, I get where your feelings are coming from - it's very different when it's your own parent. My dad actually felt ready to form a relationship about 7 years after my mum's death, and even after that amount of time, it still felt really weird on an emotional level - I was happy for him, but at the same time felt "how could he forget mum?" and also, if I'm honest, felt slightly pushed to one side (I wasn't, of course, this is me saying how I felt). And those feelings must be massive when it's coming so very, very close to your mum's death. I'm not surprised you feel hurt and confused and shell-shocked. I wish I had a helpful suggestion as a quick fix, but I fear this may be one of those "keep buggering on" situations, where only time will help (in the sense of learning to live with the loss- the loss never goes away).