Gutted, the first thing I want to say to you is to forgive yourself for losing it. That calmness and numbness had to wear off at some point, didn't it? Sometimes, when you have been through what you have, the smallest thing triggers emotions you didn't know existed.
It's sod's law that it all came flooding out at the worst possible time and in the worst possible company, but it has happened and you probably needed a catalyst to get you to this point. Living with suspicion again was probably eating you up far more than you realised. You are human, you have been very hurt and the truth is, you have not healed from the last betrayal.
In the calm after the storm, consider what posters are saying here.
In the recovery period, all of us have moments of doubt or new suspicion. The truly sorry spouse though, completely understands this and takes great care to ensure that they do nothing to provide their partner with an awful flashback or new suspicion.
Fortunately for me, these have been very few and in fact the only one I can remember off hand is one night when it was obvious that my H had his phone on silent (it never is, these days). That provided a flashback and as soon as he realised he hadn't switched it back to normal after a work meeting, he was literally prostrate with anger at himself. He understood exactly why that would give me a flashback and was so sorry.
It seems that for your H on the other hand, it is bearable for him to see you in such agony. I'm sure in his defence he was also trying to normalise things for the children and so in part, I understand why he was playing with them, but at some point in all this, he should have taken you in his arms and said "I'm sorry. I understand why you feel this way. Of course you don't trust me yet and I accept that. Of course we will go to see my locker, I understand why you want to do that."
The fact that he didn't - and hasn't made much effort at all in fact to allay your suspicions, is telling.
I agree with posters' assessments of the NYE disappearing acts - this seems odd to me. I also think that carrying condoms around at all is bizarre. I expect he'll find some reason to explain away the gift set and will probably project back on to you that you are paranoid and suspicious to think it odd that he "forgot" to tell you about another purchase.
Now it's all out in the open, take some time to decide what you want. I understand all the barriers to leaving, but flip this on its head and work out the barriers to staying.
In your situation, I just don't think I could have the clarity about leaving until I knew all there was to know. If he is cheating again, will that give you the clarity you need? If so, tell him very clearly that if he has got any respect for you and your mental health, he must come clean.
You might be different to me, but I had certain "deal-breakers". I knew I would call time if I ever felt he wasn't truly sorry, if he ever tried to deflect the blame and the greatest one of all - if he ever did this again. It helped me to have that clarity. I accept I'm in a different position to you and I always knew why I was staying - nothing to do with the children, economics, lifestyle etc. I was staying because I loved him and knew our marriage was worth saving. I also knew at heart that he was and is a good person at heart, but for a short time, did some very bad things.
Try to work out why you have stayed - this will also help your clarity.
I do think there are times in our lives when something happens to jolt us out of our inertia and say "enough!" This could be yours and it might be that you decide no relationship is worth the toll on your mental health and emotional wellbeing. Trust your instincts and think hard. Happy to be a sounding board for what could be some very difficult decisions.