I have had such a wonderful life, why am I so depressed?

I was 40 and in a marriage with too much drugs, when it happened.  A beautiful creature I can only call M to prevent her embarrassment or harm.  She came to me at my office in Tampa, and told me she was turning 18 and had a request for her birthday.  She wanted Me to be her birthday present.  I was smitten.  Without a doubt it was the most wondrous and loving 3 consecutive days of my life.  We spent most of the time in the hotel room and I have such memories.  Enough to get me by till the end of my time.  I probably should have married her, but I had thoughts that I might cause her pain and grief and I had to stop at that point.  I still think of you M and miss you so very much.