What to do.....car problems

[Hickory Hill Hoff]

Here's the story...
Went to start my car yesterday afternoon and it was "deader than a doornail" thought it was because the temp was at zero and the battery was dead, but after trying to jump start it...nothing. Had it towed to the garage I bought it at in July and where my stepson is a salesman (the "HUGE" place ), it started when they took it off the flat bed and later hooked it up to a computer where it said that a sesnor in the transmission may be bad and would have to be replaced. The cost...$600, lot of labor involved. This same thing happened about 20 years ago with the first car I had to make payments on, had that one 3 months and the transmission went on that one. Had to "bite the bullet" because at the time, there were no lemon laws in N.Y. state and my grandmother was the co-signer on the loan. It's cost me over $800 then, just my luck the same thing happens two decades later. Made 5 payments with 67 to go! They don't think it's under warranty anymore, so guess what.....
Anybody got any ideas or is a mechanic here? Do I just let it go and take my chances or fix it. I probably should get a second opinion or even get rid of it. This is the first car payment I've made in almost ten years....just drove "clunkers" but decided to get a "trustworthy" car.....see what happens!