Yaris seems to get stuck in 3rd gear when going uphill

[47_MasoN_47]

I don't know what the deal is, but I often find myself wishing the automatics had a 4th gear on the shifter so I could force mine out of 3rd. Often when I'm going uphill, for whatever reason the Yaris will get to 3rd gear and then just quit. Yesterday when going up the long gradual hill from work back to my house I was going about 45 in 3rd and it was running 3000 RPMs. I was cringing because I'm sure that undid a lot of the low RPM driving I did trying to get better gas mileage, but I couldn't slow down lest I get ran over. I tried letting off the gas, shifting back and forth from 3 to D with the shifter, and nothing would work. It didn't shift into D until I got to the crest of the hill and began going back down the other side. What's going on here? It seems like it does this on every hill I come to.

BTW 2009 Sedan AT