Oh Edward you certainly didn't fuck up. You were young, and you had already given a lot of your life and your time to caring for Dorothy.
If it makes you feel better, let me tell you I too live with regret. I should have taken leave when my mum was dying, but I didn't for all sort of reasons, mostly my work commitments as a teacher, and as a mum.
There are many people out there who feel very much the same, yet maybe they don't express it.
Look at what you are doing today! I imagine you'd be one of the most caring nursing professionals. I'm sure too you make every demented patient's life a little better each day.
Please, do forgive yourself. Dorothy already has. 💜