Sending you a welcome hug
Grief is a horrid journey, and one on which we can feel very lost and confused.
I can understand why you feel rejected and on the outside. Its difficult when theres children and an ex involved - the children will want the support of their mum, and its a challenging time for people getting on when everyone is so emotional. Your partner is reeling from his loss and may not be thinking straight/realising the effect its having on you
I know there were challenges at my nans funeral, (mums mum) my aunty (cousins mum) was not welcome/invited and new partner came which did cause some friction/difficulties. My parents had split many years ago, my dad came to the funeral whereas my mums partner of many years did not
Perhaps you can focus on why you are together, try not to push him too hard on his loss, but be there for support/care.
You could do with some support as you're grieving too, and another loss with heighten your loss too. We're here for you on this tough journey
xd