My achilles tendon is short so i can't stand on my heels, only on my foot. The problem is that it is very hard for me to maintain my balance like that and and because of it it is very hard for me to concentrate. What can i do?
You can pray while sitting, either on the ground or on a chair. A lot of people I know have arthritis and pray on a chair.
Pray while standing and if you can't, pray while sitting and if you cannot do even that, then pray Lying on your side. (al-bukhari)