You cannot hold yourself responsible for what your late wife did.
A friend of mine committed suicide a few years ago, and the night he did it, I was supposed to play a game with him on steam, the same game we played each week.
But I had a young baby at that time that didn't sleep well, so that night, I told him I couldn't play this week because I was really tired and we would play next week.
I wondered if that would've happen if I played with him that night. Sometimes, its all it takes to change the course, just as it takes only one moment of weakness to take your life.
But I cannot hold myself accountable for something that he did.
From your account, you've tried your best. But in the end, your wife did the act, not you.
You've done your best, and that didn't change the outcome. No one has the right to tell you its your fault.
Her mother lost her baby that day, but four children lost their mother, you lost the love of your life, and your family was forever changed.
It's easy to dwell on what wasn't done at that moment, but we easily forget all the efforts we've put until that day and after.
This is a terrible situation and I hope that you can find the support you need to help you get through this. Best of luck.