My mum died 8 months ago.
My dad and I are still in an extremely raw phase of grief.
Her clothes are still in her wardrobe and her shoes are still in the hall. Her side of the bed hasn't been touched by another person. Her toothbrush is still in the pot.
Yet, in less than this amount of time, your husband had grieved, recovered and met someone new (who he very swiftly married and moved in to the marital home.)
Can you honestly, truly not see how stupid and insensitive the both of you have been?
Honestly, if it were my dad I wouldn't want to meet you either. I wouldn't be making polite excuses though, I would be telling the both of you to get to fuck.
Losing your mum is quite literally one of the toughest 'milestones' (for want of a better word) to deal with. They may be adult children, but they're still the children of the mother who has passed on. To be suffering with that loss at 28, 24 and 22 must be utterly heartbreaking.
What's done is done now. I don't think you moving out or splitting up with their dad would reverse the damage that's left in your wake.
You're only concerned now because you are in a situation which is uncomfortable for you - if the kids were playing ball and it were all happy families you wouldn't be posting here. Yet, the situation would still remain. You've still bulldozed into their lives with little consideration for their emotions.
I don't know what the answer is here. I just hope the kids find peace.