It sounds like your finances have been too closely enmeshed to work out if there is anything actually “owing”. Rent is a significant amount for most people and, presumably, will be a big chunk more than you’d otherwise have needed without your ex’s children. He’s been paying for their uniform until recently and food at the beginning of the month.
You’ve been paying for a wide range of other expenses that sound more variable and since your joint DC you’ve taken the hit to any sort of career progression and, presumably, to your pension.
It’s all been very ad hoc. It does sound like you’ve been okay with him putting some expenses on the CC for your kids, but also as though you’ve been uncomfortable with the level of spending he’s been happy with. I don’t think you can really work out whether he’s funded your DC or not.
So it makes sense to look at what paying off or keeping the money would mean. Your concern about your relationship and what that might mean for you and all 3 of your kids if he walks out after you’ve paid down the credit card is obviously something a sensible mother would consider.
What do you think would, long term, be best for you and your kids?
Would you, ideally, stay with him and try and make it work? If so, I would think you’d need to pay off some of the CC but keep some of the money back. More importantly you’d both need to sit down and come up with a budget between you that would be balanced and allow the credit card to get paid off over time. That would be a working together approach. If he just wants to get himself out of debt without any sort of joint endeavor to getting and staying in the black, I think this is a lost cause regardless of how you use the CMS lump sum.
Or, have you been clinging to this relationship because you couldn’t see a way out, and now you have this lump sum you might actually be able to set up without him? (Something that, if he has actually been subsidising you would make him better off long term even if you didn’t pay down the CC).