My ex did this over many years. You are already ahead of me though in that you don't trust him and have access to the finances. I didn't trust him, but had virtually no access and no information. Only during the divorce process did I uncover the extent of his financial abuse. My regrets are:
Not manning up to the fact that I had EVERY RIGHT to know every financial detail of joint finances
Not just finding out for myself. I did not need to ask his permission for anything really, or to make him get the accounts out, I could have just waded in and found out behind the scenes. Had I done so I would have been able to confront him fully, armed with all the evidence.
Not doing the above as soon as he said he was leaving
Not freezing all the accounts any talking to lenders immediately, instead falling for his convincing reasons to leave everything ticking along for more than a year
My ex took money off me by SO every month for mortgage and bills, just like you, but behind the scenes was able to save up tens of thousands secretly since I was covering most living costs. During the divorce process he also took as much credit as he could straight out of the mortgage and trousered it, then lied to the court about it and tried to make out I was responsible for all the debts and he was saving me from a poor credit rating!!!!!
My advice is, if you are anyway thinking of leaving, find out as much as possible without his knowledge anyway, you will have to do this anyway for financial settlement and he will obstruct you and hide money if he is on to you.