Ford :: 2007 - Can't Get Into Gear - Shifter Won't Budge

I drive a 2007 Ford Focus (manual trans), and it's been acting funny lately.

Here's the problem: Lately, when I start the car and try to put it into gear (either 1st OR R,) the shifter won't budge. While the engine is off, I can put it in gear just fine. After I wait for a little, turn the car off and on again, etc, I can get the shifter into gear if I really push it HARD. (I know I probably shouldn't do this, but... I was terribly late and desperate!) Okay, so I get the car into first and manage to pull out of my parking spot without having to reverse. When I go to get into second gear, I've still gotta push the shifter REALLY hard (again, bad!! I know!) but maybe a little easier. Where I live, it's stop-sign city, so I have to stop and go a lot, and within a few minutes of this (maybe 3 or 4 cycles of N, 1, 2, maybe 3, N) I can shift smoothly again and the car feels totally normal.

The strange thing is, this happens sporadically, and I can't seem to see any external factors that correlate with the problem, except that it seems to be worse when the car's been sitting for a couple days. Sometimes the car shifts just fine, and sometimes it won't get into gear at all. In general, the car's in pretty good shape: no accidents, scheduled maintenance, etc. I guess the only caveat is that a couple years ago, TWO people learned to drive stick in this vehicle, so maybe the transmission has taken some abuse.