nissan sentra transmission not shifting correctly

2015 nissan sentra 30k miles - A little over a year ago I noticed the car seemed to be getting more and more sluggish and that i was getting less and less mpg, (went from average 32mpg down to 16mpg over a short period of time). It was not even near due for tuneup but i wanted to nip this in the bud. Also the CVT didnt feel like it was shifting correctly but at that time I wasnt aware of the major catastrophy the CVT so I was going to fix one thing at a time. Just before i gave it a tune-up, the check-engine light came on. It was the code for the "mass airflow sensor" problem. I decided to just replace it and be done with that. Also do the tune-up at the same time. Replaced spark plugs & wires, oil & oil filter, air filter & pcv valve. Also reset the code. Check-engine light was gone but came back about a week later. So I took it to the dealer. Explained everything I did and left the car with them. Dealer calls me back later and told me they would have to keep the car for about a week because they found problems with the trans and they have to replace it and that it would be covered under warranty. Week later I picked it up and they said they only needed to replace about half the trans. Also they cleaned up the throttle body and reprogrammed the computer with updated software. About 3 weeks later the car was soo much worse. my mileage was down to 10 - 12mpg and it felt like I was towing a locomotive behind me. When I would start to go after a red light, I would have to floor it to slowly get going, (a moped could easily beat me in a race). Then check- engine light again. I took it back to the dealer, a couple hours later they called me to pick up the car and that it was fixed no charge. When I picked up the car they told me it just needed a little adjustment and update the software again. I protested because by now I educated myself about this car. They insisted I take it for a test drive with their mechanic. The car wasnt acting up real bad the way it was earlier but I could feel it was a little sluggish. I told the mechanic and he was adamit that it was working perfectly. I tried to convince him but he wasnt having it. So I left. Within a couple hours the car was getting 15 mpg again and check eng light back. They accused me of driving it to hard. I made another appt for a week later. As i was still using the car the the check eng went off then couple days back on. when I took it in, they called for me to pick it up. I asked what was wrong this time and they said nothing and did not find any error codes. I needed a car and did not have time for this so I left. Short time later the car acting up real bad. I could not get the car up to speed on the freeway, car would rev up to 4500rpm just to get it to move from dead stop. Also would stall. I took it back again and they called me later and told me they could find no codes or problems. They wanted to keep it overnight to test drive it some more. I said fine but I needed a car. They refused a loaner and told me to rent a car. I paid out of pocket for a rental. Next day, same answer, "the car is working perfectly". Now I am driving a defective car and the dealer wont fix it because according to them "there is nothing wrong". The car has just barely 30k miles on it and it is useless if I cant drive it saftly. I cant sell it in this condition. What do I do now?

Nissan CVT transmission are not the best and I tell customers to avoid the brand. Most times replacing the transmission can get you another couple thousand miles before failing again. I know you don't want to put more money into the vehicle. You can have an indendent trans shop look into your issue, costing you more money. Also the current dealer knows of the issues, so yes, you can't trade it in without taking a big hit. I suggest you find another brand , Toyota or Honda and trade in your Nissan. Taking a smaller hit. But most times dealers today will only check to see if there is an engine light on and drive it around the block. Good Luck.