He hid $412K while I skipped my sister's wedding

AITA for opening my husbands retirement savings account and finding out our entire marriage was funded by someone else. I am thirty four years old and I have been married to Mark for seven years. We live what I thought was a modest but comfortable life. He works in logistics, I teach elementary school, and we always joked about how careful we were with money. Last week I needed to check something for our taxes and I logged into an account I thought was our joint savings. What I found instead was a retirement account in his name only, with a balance of four hundred twelve thousand dollars. I sat there staring at the screen for almost ten minutes convinced I was reading it wrong. We have been pinching pennies for years. We skipped vacations. We ate rice and beans the last week of every month. I confronted him the moment he walked through the door. He turned pale and would not look at me. He finally admitted the money came from his mother, who passed away three years ago. She left him an inheritance and he never told me. I asked him why he would hide something like that from his own wife. He said his mother made him promise before she died that the money would stay separate, untouched, a safety net only he would control. I felt like the floor dropped out from under me. Seven years of marriage and he let me believe we were struggling while he sat on almost half a million dollars. I asked if this was about control or trust and he just kept repeating that it was what his mother wanted. I could not understand how honoring his dead mother meant lying to his living wife every single day. We have shared every bill, every stress, every moment of financial fear together, except apparently none of it was real for him because he always had a parachute. I started going back through memories in my head. The time I cried because we could not afford to fix my car and had to take the bus for two months. The time we skipped my sisters wedding because of the cost of travel. He sat beside me through all of it knowing he could have solved it in one phone call. I asked him point blank if he ever intended to tell me. He hesitated so long I already knew the answer. He said eventually, maybe when we were older, maybe never. That word never broke something in me. I have spent years building what I thought was a partnership and finding out it was actually a performance is devastating. My sister says I am overreacting because at least the money exists and nobody spent it on an affair or gambling. My best friend says secrecy on this level is still a betrayal, regardless of where the money came from. Mark keeps saying I am making this bigger than it needs to be, that nothing has actually changed since I did not know about the money before and my life was fine without it. But that is exactly the problem. My life was built on a version of reality that did not exist. Every sacrifice I made was voluntary only because I did not know I had another option. I keep thinking about how easily he lied, how smoothly he kept this secret for years without a single slip. It makes me wonder what else he could be hiding, what other version of reality I am currently living in without knowing it. He wants to move past this quickly, says we can start using the money now that I know, as if that fixes the years of hidden stress and pointless sacrifice. I told him I need time to think, and he looked at me like I was the one being unreasonable. I do not know if I can look at him the same way again. I do not know if trust can survive learning that the person closest to you decided, for years, that you did not deserve the full truth. My sister keeps asking what I am going to do next, and I honestly do not have an answer yet. Am I overreacting by feeling completely betrayed over money that technically never belonged to me in the first place.