SUBREDDIT: r/relationships
TITLE: Me [24 F] having trouble with my mother [50 F] moving on too fast after father's [60 M] death
POST: My father passed away very suddenly in a car accident in October and have been having a really hard time with it. My parents had been together over 20 years and had an amazing marriage. Very loving and caring family and my father had been a great man. 

I noticed a couple months ago that she was hanging around someone from their friend group a lot more. I made the idea of me being uncomfortable seeing her with a man well known. I was told that there was nothing going on with anyone even though I suspected it. Flash forward to about a month ago when she tell me she is dating this guy. 

It makes me sick to my stomach when I think about them together and have been more depressed than usual the last while, oversleeping and more irritable. 

I've approached the subject a couple times but it seems like a lost cause since I hear the same thing every time. "I'm not replacing your dad" and "I never expected this to happen". I have some other siblings and they all live at home, while I'm the only one to have moved out, but still find it really inappropriate to not have a sit down discussion of how we would feel about her dating. We used to have a very open communication between us.

Now they have made their relationship "facebook official" and can't even talk about it without getting really angry or upset. I know that everyone deserves to move on and be happy, but less than 6 months after, considering how sudden the death is. I feel like this is something I could never forgive her for and it's eating away at me.

Apologies very the rambling and unorganized post, it's hard to put my thoughts into words.
TL;DR: 
I [24 F] feel like my mom [50 F] is moving on too quickly after dad's [60 M] death. Any one go through anything similar, any advice and what I should do?