I rode my most difficult ride a couple of days ago and it was great. I rode about 45 miles and near the end of the ride was the biggest/steepest climb of the day. The moment when I was feeling the most tired was when the biggest climb was approaching.
You can wince and groan that the difficult part is happening or you can attack the hill. Find your limit, don’t impose one prematurely. Attack the hills and enjoy the challenge. It hurts because you’re getting better.