Honestly, trying not to beat myself up too much is a big part of it. This sounds like a wishy washy answer, but for real, I improved a heckton when I started giving reasonably accurate updates.

E.g Before: I have arranged to meet with a friend at 4pm. My travel time is 30 but can be variable due to traffic. It takes me 30 mins to get ready. 3:15 rolls round and I still haven't started getting ready. If I hurried, I might make it in time. I do not do that. I cringe internally and end up indulging more deeply in whatever distraction caused me to overrun. 4pm rolls around and my friend messages to check in. Either I cancel, or I tell a lie and start getting ready, hating myself all the while. My friend is irked at me, and I don't blame them.

After: The same as before, but at 3:15, I message my friend to tell them I'll be half an hour late (I round up to account for being bad at time). I end up being 5 or 10 minutes late nonetheless, but my friend isn't annoyed, partly because I kept them in the loop about my progress. I still cringe at being late, but I find that over time, I get better at genuinely holding myself accountable, and at estimating time.