Q: Just changed thermostat now my car is running hot worse than before, I think I need to check if the new one is opening up?

Car was running hot before but not to the point of over heating. Mostly at slower speeds, once on the highway it would level back out to about 180-225. From there I was able to run the A/C, back to 40mph and below had to turn off A/C. I don't know where I went wrong, from my research I assume the new thermostat is stuck closed. If that is the case how would I fix that? I changed the plastic T piece connecting the hoses to the radiator and antifreeze reservoir so there's no more leaks. When priming the car it ran hot and the person helping me wanted me to just let it run even while hot. The car started shaking and said low engine pwr and change oil soon, although it's got a fresh oil change and plenty of oil remain. They said under the hood it hadn't run hot at all, not even bubbling. What else can I try?

My car has 196443 miles.
My car has an automatic transmission.