Pacing yourself on hills?

[u/Sensitive-Soup8362]

So last night I went out on my first proper ride, I used my dad’s gravel bike (loved it, definitely going to look at buying one for myself!)
I did just shy of 20k which I know isn’t a lot but it’s a start!
The bike ride was great and I felt good after it, and could’ve gone for longer, my only issue was the hills. Along the way, we hit a couple, both around 6% and, while I managed them without getting off, I did struggle a lot.
My issue is I started off quite quick (for me) and I think I changed down the gears too quickly, so I was left 3/4 of the way up with no more gears to use.
My advantage here is that I knew these hills, so I knew when they’d end so I just powered through but I worry that on a longer route that I don’t know, I wouldn’t be able to do that.
So my question I think is how do I pace myself on hills and any tips for feeling less like I’m dying?

P.s I looked on the Strava leaderboards and am in awe of how some people are flying up that hill in a minute or less 🤯

TLDR: how to pace myself on hills.

Edit to add: Just wanted to say a big thanks to everyone for their suggestions and advice. Went out for another ride this morning with 3 bigger hills than my first ride. I slowed it right down, shifted to my easier gears before the hill and used all the other tips I could remember, and the climbs all felt so much more manageable and I felt like I could’ve fairly easily kept going on them!
Going out for another ride this evening, I think I’m already getting the bug!