Get a new trimmer.
I did a lot of lawn cair maintenance through high school and my early university days. It was not uncommon for me to be seen smacking the stupid weed eater against the ground in a vain attempt to make more cord come out.
Last year, however, I went out and got myself a new electric weed eater. I had only used gas ones before but heard good things about
this model. I've only been using it for a year and a half but it's been great. No hassles with the cord at all. It keeps coming out magically without my intervention.
The only real downfall is that since it's battery-powered, you'll sometimes find yourself halfway done your yard when the juice runs out. Luckily the package I bought contained two batteries so one is always ready to go.