Hi Elizabeth!
You are not alone in your quest to find out what the heck is happening and why am I not getting the same results I once did. It is very common yet can be very complicated at the same time. Main reason being, not only does our body slow down as we age, our priorities shift and we don't have the time to just focus on just ourselves now. Many are mom's which pretty much demands your time anywhere form being a diaper changing service to being a taxi cab service for our kids...
We have our full family to focus on,
Another thing that starts to happen as we get older is our bodies shift to pre menopausal state and then extend further in to full menopause which can go on for years. At this point our bodies aren't producing the same hormones we once had so therefore our metabolism will shift and our waistlines thicken.
Also the aging process itself is a whole reason for us not getting the same results. Our cell turnover slows down and our muscles atrophy and aren't capable of building the way they once were, our bones get brittle, our ligaments and joints weaken, etc. Our metabolism is effected not only by our workouts but by our foods.
As we get older our body is less capable of metabolizing carbs and filtering the "yuck" that comes with processed foods. We expose ourselves to possibly develop rashes, inflammation, food allergies, and other conditions that result from the body's inability to process foods and release toxins adequately. All of these things add up to slow us down.
Depressing huh? Well let me tell you, knowing all that, it is not time to throw in the towel. Exercise offsets the aging process so you must continue but just with a wiser approach. Doing doubles or going longer and harder might only invite injury and further set backs. You'll have better results with longer warmups, longer cooldowns, longer recovery between workouts, include stretch workouts weekly, and eat clean, super clean!!!! You must respect daily aches and pains and modify all workouts accordingly. You must respect injuries and let them heal while doing physical therapy to build your foundation back up slowly and gradually.
When I was younger my warmups ranged from 3 to 5 minutes. Now I love a good 12 minute warm up. Incorporating yoga twice a week has really kept my joints stretched and limber and helped to reduce stiffness around my joints. I need two full days between weight workouts after a particularly DOMS related workout. Enjoying the post workout cooldown/stretch verses flipping from stretch to stretch has made a world of difference too.
Hope this helps you