I apologize ahead of time for my lack of knowledge..

This all started when i added regular coolant instead of G12... major newbie mistake.. I wanted to die when i saw the ge-lo in my coolant tank. I flushed it 3 times since then to get the ge-lo out.. 2 days ago my car started to overheat and i realized it was low on coolant so i added more but it still heated up.. i finally found the culprit.. the ring around the coolant tank was oval instead of round and my cap was no good anymore.. because of the loss in pressure it would heat up and start to drop coolant from exit valve on the bottom of coolant tank. I bought a new coolant tank and a new cap which solved heating problem.. As i was testing it out i noticed my car would no longer go past 3rd gear and my tiptronic doesn't work anymore either. 1st gear goes all the way to 3,000rpms before it switches to 2nd, 2nd gear goes to 2,000rpms and 3rd gear kicks in instantly after. I called an audi repair shop and the mechanic i spoke with said he would need to rebuild transmission and it would be around $2,800. He said he just finished rebuilding one on another A6. I don't have that kind of money laying around so i came here hoping for your opinions and knowledge. I never had a problem with my transmission till 1 day ago when i switched coolant tank and cover..
History for the last 2 days..
1. car started to overheat to the point where the gauge showed hot 1 second and dropped to cold. Had just pulled into driveway as it happened.

2. Found coolant tank and cap was no good so i bought new ones and installed.. no problem there.

3. tried to drive car.. no more overheating but it wont go past 3rd gear.. i ca go up to 35 mph without forcing engine.

If there is anyone around my area i sure could use some help.