No. You're thinking isn't straight on this. You're trying to re-engineer the past based upon an outcome that you know now, but you didn't know back then. You can't do that and here's why. Had you took her money and you doubled it for her and her family within a year or two, you'd feel like a genius today. This thread wouldn't even exist. But because you had an unexpected outcome that created some trauma, you're now living with periodic regret and guilt, despite the fact that your intention at the time was certainly not to lose her money, your money, or anyone else's money.
When you say things like, “I” could have done something about that situation, or “I” knew better, then that’s just your ego talking and ego absolutely loves guilt. They are best friends. Your ego wants you to believe so bad that you were such an awesome and amazing person that you could have prevented that tragedy if you had just taken that call. But you can't let your ego control your thoughts, because if you do, you'll start believing those thoughts are true, and when you believe them to be true and real, they have power over you.
Never forget that a thought about the past, whether bad or good, is just a thought. It's not relevant to today. The essence of who you are doesn't lie in the past.
The only thing that you ever truly have is "now", and it's the now that's worth thinking about more than anything. That's why my suggestion was to write down some things you're thankful for today. It helps pull you out of the regrets of the past and refocuses you on living in the moment.