I'm perpetually late. Even worse, I completely stress out about this often. So, I've been trying a trick from Greg McKeown's book, Essentialism: Build in a buffer. Pretend that everything is going to take 50% longer than you predict.
The research shows that this buffer is relatively accurate. For example, psychology students were asked to estimate how long it would take to finish their theses (averaging 33.9 days) and what it would look like 'if everything went as poorly as it possibly could' (averaging 48.6 days). The actual average time was 55.5 days. We overestimate what we can get done and underestimate what can go wrong.
I've been trying this for the past two weeks. My stress levels are way down. I didn't freak out when Rowan had an accident at drop off, or when I had eight people over for brunch on Mother's Day. I had time.
The surprising thing is that 50% isn't much of a buffer at all. It's been an accurate predictor of how long things take.
So, give yourself more time. Yes, that means you need to do less, but that's the whole point.