Rule #1 rears its ugly head again....never believe a salesman when they assure you that you can tow just fine with the vehicle you have. Something about let the buyer beware. AKA do your homework before you sign. And if it's any consolation, 'tis the season and you're not the first with this problem this year.
So let's assume you're not quite ready to make a change in tow vehicles. Suggest the first stop you make is to your dealer and if you don't have one you trust, ask around until you find one...or have this chat with several but don't make a commitment yet. Find out what changes you might make to your existing truck and what it's going to do to benefit you and the cost. If you have to, call the manufacturer and talk to them. Bummer that it is, not everyone is in a position to just jump out there and take on a $400 or more vehicle payment. You may have to do some tweaking to what you have unless a new truck is on your shopping list.
The other option would be to contact the dealer you bought the trailer from and explain that you were given inaccurate information by the salesman and see what might be done to swap to a different model without taking a beating. They're not likely to admit one of their salesmen was "wrong" but you have nothing to lose.
Sorry you're finding yourself in the middle of this situation. I hope you can find a relatively easy and reasonably priced solution. Remember #1,