1994 Nissan Sentra Popping out of 5th gear

Hello
while driving along my engine RPM went very high up when I noticed that I had popped out of 5th gear. When I put it back into 5th gear it pops right back out. When I put it into 5th gear and hold my hand on the gear shifter I can feel it wanting to pop out. When I shift into 5th gear I have no problem shifting into it. What problem do you feel I have and is it worth fixing or relacing?

Fatz