I don't usually regret the things I do in life because I'd like to see them as life lessons to be experienced and learned from.
A little story before I say what my regret is. My dad and I used to be really really close. I was his little girl even when I was a teenager, he was always so protective of me. Even though they were done in subtle or weird ways, I was always sheltered and cared for by him. But during his last few weeks, we weren't on good terms as there was a misunderstanding between us. He wouldn't hear me out and stayed stubborn about the fact that I was wrong (but I really didn't do anything :\ ).
So, there was one thing I did regret. On the day I had to call an ambulance when my dad wasn't responding, the paramedics told me I could not take the ride with them in the back as for some reason, they couldn't allow that. I regret that I did not force my way up the ambulance in the back where they wheeled my dad in. I regret that I couldn't hold his hand and be by his side when he gave his last breath, when I couldn't ask him for his forgiveness for the things I've done wrong. He went all alone. Just as he had predicted he would have. I regret all that and for a while I took it quite hard. Couldn't believe I was so stupid. But I guess I just wanted him to be okay and get to the hospital fast, didn't want to hold them back by arguing - or I was just in a daze with all that happening.
That's the one thing I regret in life. Not able to be by my dad when he passed on in the ambulance. :\