I can't help but start with one of my favorite Zen monk stories. I'm sure it's horribly mangled from the original, but this is the version I know, and it goes like this:
One day, three Buddhist monks are traveling the countryside. They come upon a stream swollen from recent rains. The banks are slippery and muddy and on the bank are two women dressed in fine clothing, peering suspiciously at the mud. They see the monks and imperiously demand to be carried across. The monks do this, slipping and sliding and dirtying themselves still further. Once safely on the other side, the women do not give alms, nor even thanks. One scolds the monks for letting her hem get dirty.
The two groups go their separate ways, and two of the monks begin to commiserate about how terrible the women were, how unpious and ungrateful. After listening to this for a while, the third monk turns to the other two and says, "Brothers, I put those women down on the bank of the river long ago. Can you not do the same?"
The meaning, as I take it, is that bad things will happen. People will behave badly. But it does us no good to carry around those ills and hurts forever. The things that happened? They happened. Start from where you are.
Earlier I talked about the arc of past selves that comprise (peoples' image of) me. There is certainly a good deal of my past I'd like not to have gone through, things I've done that I regretted. The normal human stuff. But if you offered me a foolproof time device to go back and change that stuff I would probably turn the offer down. Start from where you are.
Partly this is about unforeseen consequences. If I was not who I was I would not be who I am and I'm not sure who I'd be instead. It feels better (and yes, safer) to start from here and work on future me rather than trying to fix past me.
So here I am: I have this body, this life. It has its good and its bad and I can spend plenty of time complaining about the bad things (let me tell you about my allergies, or how much arthritis sucks). But it's where I am, and I'm starting from here.