How can I manage to do everything I once did - or can i?

Proper expectations. It is important to have proper and appropriate expectations. Although it is certainly ok to want to do things as before, it may not be appropriate to expect the same level. Although i would like to ski like before, now I am just happy to be skiing. Set the right expectations.
Energy. You are very young to be limiting what you can or cannot do, check with your doctor to find out what is limiting your performance, it is unlikely to be your age. Best.