transmission warning light is on


i just recently bought a 2nd hand bmw 1996 M3 325i... The transmission light suddenly showed on. When it’s in the drive mode after a couple of minutes running, the gear is not shifting after even pressing the gas accelator harder.. I changed the atf fluid and the transmission filter already but still has the same problem.. when i changed from drive to neutral mode there is like a hard slide..

please advice what to do. I have asked a Mechanic who is familiar with transmission and I was given an estimate of around $2000-2,500. This is really a hefty amount. 

Looking forward in hearing your comments and advised.