@loutypips my situation is exactly the same although in my case it was my grandmother, not mother. She raised me though so was like my mother and best friend rolled into one. I had avoided visiting for a week as I had a bad cold and knew she was immunosuppressed. Then, when I felt well enough, I went to see her but she was asleep. I sat for a while but didn't want to wake her as she looked so peaceful so eventually left to go home. The call came early the next morning to say she had died and I'll never forgive myself for not waking her to have one last conversation or at least letting her know I was there after not seeing her for days beforehand.
On a more lighthearted note, I regret not taking the opportunity to have one further fling with my ex boyfriend when I was 16. I had fancied this guy for ages and was over the moon when he asked me out but our relationship didn't last long. Then one night, we were hanging out in a group of friends but he kept flirting with me and we were kind of splintering off on our own. I stupidly decided to play hard to get and really overdid it to the point where he gave up. We never had another connection like this although I remained deeply in love with him for another couple of years. I am deliriously happy with my DH and our life right now- but I still wish I'd had another go with my first love.