I have a honda civic 1.6 P reg. oil and water is fine but the temp gage is always nearly full when I drive longer than 20 minutes, I can bring it down a bit when I put my heater on full. I checked the temp sender and the fan works so do you have any other ideas?

Replace the radiator, since it's likely corroded or blocked by phosphate scale. Afterwards, fill with a 50/50 mixture of G-05 antifreeze and distilled water.

Have you checked that the thermostat is opening? And is the water pump working? I'd check the thermostat first though