Transmission Gear Slipping problems of the 2019 Nissan Sentra

I was trying to merge into the highway traffice and I pressed on the gas pedal to pick up speed, but the car just puttered along with the rpm at around 3000 - 6000 and I almost got rear-ended by a van who got really close to the rear-end of my car. The gentleman in the van was clearly upset and I know it was my fault because my car did not want to pick up speed. I have owned about 80 cars in my life and I have never had this problem; I know how to merge into traffic very well, I have been driving for 40 years without any accidents or tickets. I took the car to charlie clark Nissan dealership where I bought the car and they said there is nothing wrong with the transmission. This car makes me look like I drive like an old lady, people are getting upset, as it has no pick up power at all even though the rpm is past 3000. There is just no way out of a potential dangerous situation. The reason I think it is the transmission problem is because when shifting from 2nd to 3rd it losses power, like when the transmission is slipping.