P - I am so sorry this is happening right now, but am glad you have your mum and friends alongside you. No matter how painful it is now, it will pass, the sun will come up tomorrow and you will still be a brave and fabulous woman with 2 children that love you
He’s messaging me asking if I love him and do I want to fight for my family.
This is an interesting move from him - I suspect he is consciously or unconsciously trying to sabotage you.
You are making progress, have a big promotion in front of you, and hes thrown the biggest emotional spanner possible in the works of your relationship, with pretty perfect timing to derail you - and is demanding you focus on him/fight for your relationship at exactly the worst time for you, like a toddler throwing a tantrum for attention
Does he feel threatened by you and your success? Is he afraid that your focus will move to work more than centering him/your relationship? Is he worried about losing the role of higher earner/big man of the house if you are equally successful? Is he resentful that his life will change or need reorganized because of your new job (e.g he might have to do more domestic work or parenting?) - having had an ex DP who tried his best to undermine my career plans and would inevitably create drama in the run up to any big exam or interview Im a bit wary of the manipulative games weak men can play.
Thinking about his motivations and spend time analyzing them is probably the last thing you want to do: but the claims (incredibly young love interest, haven't slept together, just emotional but I am going to blow our lives up for it, do you love me/do you want to fight for us? ) suggest a lot of neediness rather than straightforward 'I'm moving on/walking away'. He wants you to do the 'pick me' dance, focus on him, fight for your relationship, like a test of sorts. If you do it, hes winning, and if you don't do it, the breakup becomes your fault.
Either of these reasons behind his behavior are pretty much the actions of an unmitigated shite with no care for his wife or children: and I hope that you can move on from his manipulations.