The other posters have given good info. I just wanted to add to try setting the fan to on instead of auto so it runs all of the time. This will help keep the air moving in the house and might help keep it cooler. Also make sure you check the filter and change it if you haven't yet. See if your energy company offers an energy audit. that will tell you if you have leaky windows, doors, or insulation problems. I have shitty insulation but with the ac fan running all of the time I can keep the house @ 70 - 72 on a 95 degree day.
I'd also consider getting a second opinion from another HVAC company, you could have undersized ductwork or another problem that the first company isn't acknowledging.