What object do you still regret losing either because of its material value or for sentimental reasons?

Not object, but objects...a collection of various action figures that I had collected while growing up and stupidly entrusted to my so called "brother" (who made but didn't keep a promise to me to take care of them, he traded them all away practically immediately🤬). I don't know how much material/monetary value they'd have now though I'm sure it would be some, but they had a heck of a lot of sentimental value to me.

Allen Iverson rookie card, picks from high school days

Laptop or phone

I don't get attached to non-living things nor have I ever owned anything which had sentimental value. That would be foolish.