Getting things done happens one step at a time. I rode 70 miles with a group a few weeks ago, and they decided a long climb needed to be part of the ride.
I had no idea how long the hill was, and in situations like that I pull back pace to ensure I can finish and still be successful on the ay home (and that point we’d only gone about 30 miles).
The secret is to just keep pedaling.
It doesn’t matter how slowly you’re moving forward, as long as you are. Keep spinning those pedals, and put it in a gear where it is easy to pedal.
Just keep pedaling. Turn it over and over and no matter how tired you are, no matter how steep the climb, you’ll still make forward progress.