Hard shift into second gear

I have this low mile car listed above it is the Cartier model. I shifts hard into second gear and only second gear. There is no slippage when shifting into any hear. Up-shifts and downshifts at what appear to be appropriate speeds.
Took it to a transmission shop and they told me it was"internal" to the transmission and it would need a rebuild. Unfortunately, this car also has a heater blend door problem that will cost about $1,000.00 to repair, so putting an additional $2,000.00 plus into a transmission rebuild would put me way over the value of the car. It as a very nice car, but not worth putting that much money into it. Is there something else I can try? Change the transmission fluid and clean fluid and put in some type of after market transmission additive? Would doing a tranny flush help? I get a little concerned about doing that on a car this age, because I did that with a transmission on an older truck that was working perfectly and it then started to slip.
Thanks