You can’t do what you used to do; stop comparing the current you to the old you. What you used to do was great, but the current you, for whatever reason, can’t do that anymore. You may be dealing with a disease, being older, being out of shape, or something else. The point is that the past is in the past; comparing your old self to your new self doesn’t change anything, but it may make you miserable. The important thing is to do your best, whatever your best is at the current time. Accept where you are now.