Speaking from experience (my exH), it is possible to run up quite significant debt, in his case €35k nearly 25 years ago, just on casual thoughtless spending. He bought, coffees, lunch, spent on nights out, meals and so on, it was all lifestyle. This was before we were married, and we didn’t live together. I hadn’t a clue. We earned approx the same & I assumed he was much better with money than I was. 🙄
I found out 6 months into our marriage when the mortgage payment bounced twice. Stupidly, I sorted it all out for him, consolidated all the debt into a loan that took 5 years to pay off & really impacted our financial situation.
On the money side, his debt is unforgivable - regardless of what it went on, he shouldn’t have allowed it accumulate. However, how did you not have clear sight of income, expenditure & costs, to know what the household spending was? Not to kick you when you are down, but you have also been reckless.
The next step is full disclosure. Every single document, statement, loan agreement etc. You need to find out where it went (and where the money came from). He needs to be open & engaged.
I won’t advise on your relationship as it depends so much on how DP responds & what’s behind this. Poor household management by you both is one thing, wilful misspending on something unacceptable, another.
FWIW, for the duration of our 10 year marriage, my H spent money like water. He never got better, it was never on any ‘thing’, he just wanted a certain kind of lifestyle & didn’t want to budget or limit himself. This got much worse when we had 3 DC quickly, both of us experienced redundancy & he had a period of ill-health. Our marriage ended finally when my youngest was a baby & after much abuse, his financial recklessness was part of that abuse (we are finally divorced & now apart 13 years, he still refuses to approach money ethically, lives a grand lifestyle while I have to drag money out of him & I still struggle & bitterly regret spending all my savings over the years (while married) rescuing him from his mess.
So sadly, I would say there is a significant chance his behaviour will not change, and you may have to make a decision about remaining with him.
But for now, focus on his spending (and your own attitude to money)