It's gone, so no point in worrying about it. I make sure I don't waste time by planning and executing those plans.

It depends on how you define ' waste'. Is doing nothing a waste of time? 
Sometimes, I lied on my bed all day but I didn't feel waste of time. 

If it's gone, it's gone. No point worrying about it. I just make sure I won't make the same mistakes again. 

When I look backwards, and I find that I wasted most of my time. If you have a goal, then you will not feel much about wasting time. Otherwise, the first of all firsts is to find a direction.

sometimes it makes me feel disappointment and sometimes gives me a push to go ahead and think about future