Been there. Rather recently.
I got stronger by following this beginners training plan;
https://www.cyclingweekly.com/fitnes...eginner-153317
I had to Google and read up on a lot of the terminology, it was all Greek. Bought a $30 Polar heart rate monitor, which really helped me keep a less intense pace, especially during the warm-up and cool-down parts of the ride. Also a handlebar mount for my phone to see the heart rate, and the free Strava cycling app to track progress. This training plan is truly for us beginners, you only measure against yourself.
When finished with the training block, I take a several week break, riding just for fun, then decide if should repeat it or move on to slightly harder block.
P.S. I see the Polar HRM has increased in cost, there are lots of others.