I think you have answered you own question.
The longer you have owned the car the hotter it seems to run.
I bet you have some corrosion build up that is restricting water flow.
I would knock and easy to replace soft plug out and see if the block is getting clogged up.
Another thing when you mix antifreeze are you using tap water or distilled water?
Tap water will contribute to corrosion over time, and distilled water does not and it is only $1.00 a gallon.
