Hi Emmi
I am another one who sides with you and state that everyone asking you to be the 'mature person' in this scenario who feel the same if they were actually in that situation!
My advice is that your current partner will make you out to be crazy and insecure unless you give him a taste of how this feels for you eg do the following
- Become much closer to your old family unit!
You say you had a great relationship with your ex MIL, so call her more often in his presence to see how she is doing and encourage your children to do so. Send her Mother's Day cards etc
- If you and your ex are as amicable as you say, if you can stand to attend a few events with him eg school plays, birthday parties etc just do so and let current partner know that after all you also have a great co-parenting relationship
- Ensure that your children buy their Dad a good Father's Day card/ present etc with your support
- Perhaps ask your ex for advice to fix car, or something in the house, new partner may view his role in the relationship as also being 'undermined'
All of this may be over the top acting but it may drive the point home the point when he is on the outside of another's family unit
My question is why do people commit to new relationships when they are still fully signed up to the old?
Yes I know that children complicate things at times, but perhaps OP's partner is the one acting childishly, wanting to have his cake and eat it with both units, then running off in a screaming strop when he doesn't get his way!
OP you made things too easy for him by always stepping back when it wasn't comfortable for you.
What plans are there to live together as the main unit soon etc, it may take you both to compromise with jobs/ schools etc if you want to live together
Without the above, everyone around you eg his family and maybe even friends will always see the ex as 'his wife' and you the bit on the side.
I am one for being honest with myself rather than treading on eggshells regarding important issues, only you know what you can take!
If a relationship is not meant to be for whatever reason, so be it, don't loose your sense of self worth during the process as you never know they may end up back together, which is your greatest fear!
But if you remove yourself from it, and tell him to be 'fully' with his ex as that's what it appears he wants, you may find that without you being there, she/ he also looses their appeal, as there is obviously a reason why they broke up in the first place!!! 