The past root of the verb 'to play' (for musical instruments) is نَواختَن. So if you want to indicate you played the guitar in a time in the past (like yesterday), you can say: من دیروز گیتار نواختَم: I played the guitar yesterday. But this is not the way we say that it was my profession in the past. You have to use past progressive in this case: مَن گیتار می‌نَواختَم (something like 'I could play the guitar). The present root is نَواز; so you can say مَن گیتار می‌نَوازَم (which can be used to indicate both progressive and profession). It is noteworthy to remember that sometimes the verb زَدَن (past root) and زَن (present root) is used in speaking to describe that you play a specific musical instrument: من گیتار زَدم. مَن گیتار می‌زَدَم مَن گیتار می‌زَنم.