Oh God OP that is rough.
You have had to endure what I hope and pray is something I'll never understand and have to go through and losing your husband suddenly at such a young age and with young children is more than I even want to try and imagine.
There is never a time or appropriate length to leave before starting a new relationship with someone whether it's someone as young as you or someone in their 70's or even 80's that has found companionship and a little ray of hope and happiness again.
That your Mum is trying to blag you down and guilt trip you is appalling - I'm amazed you haven't already given her the business and ripped her into next year. Not defending or excusing her behaviour but I'm assuming she is from the older generation and lives in a world where woman should marry and lay with one man and one man only and if / when he dies, you should spend forever grieving horrendously and going nowhere without a black net covering your face.
I'm being facetious of course but in the same breath – I'm also aware and have met people like that.
It's easier said than done and will take a while for you to get enough up and tackle the shitters with confidence but God in Heaven woman - you have been afforded the blessing of a second chance at happiness and you deserve it, should take it and stop second guessing, questioning whether you're doing the right thing or not. If it's what feels right for you and this guy makes you happy – it's right.
You are not doing anything that takes away what you had with your husband and the father of your kids. Your kids may well surprise you and be more adult about the whole thing than most adults.
Don't beat yourself up, dwell and feel guilty for having dealt with your loss by just going off the rails and doing things you'd never normally do because your situation was and is by no means "normal" or comes with a standard manual and textbook.
You had a blowout, you met someone that sounds like a decent guy and you absolute deserve to let him into your life and not feel bad.
My advice would be still take things nice and steady away, speak with the kids about it in a casual way they will be able to understand (wouldn't have a sit down and make it a thing I find kids are way smarter and more resilient and come up with the most incredible shit when you're just chatting in the car or putting out recycling)
The kids need to know none of this changes things and how they feel about their Dad / means you love him any less which I'm sure is the case already. Their feelings and thoughts do count and need hearing out of course but I think given time and if things are taken slowly, they will possibly come round a lot sooner than you think. They also deserve to be happy and are absolutely entitled to like and welcome him too.
Once the kids are aware and have had chance to get their head around it / meet this guy casually over a McDonald's or whatever, you can tackle and take things at a pace to suit you and from that point on – nobody else's opinions, thoughts, comments and remarks are worth a shiny shit.
Honestly I think anyone including the arsehole comments on here already that really thinks you're doing the wrong thing is either just bitter and unpleasant by nature or not quite a full set.
Let yourself be happy. This is gonna be the year for you to start picking up and starting over and you can be happy.
Giving you some daffodils cos they make me super happy when I see the little green shoots appear that says Spring is on its way


I want updates on this so please keep us posted.