Low flow toliet keeps running or doesn't fill - how to fix?

Yes you can fix yourself. Here are a couple links to help you out. Pat yourself on the back, they charge at least $100 (typically much more), for a service call of this nature where I live.

http://m.wikihow.com/Fix-a-Running-Toilet

https://www.todayshomeowner.com/video/repairing-a-running-toilet/

https://www.familyhandyman.com/plumbing/toilet-repair/how-to-stop-a-running-toilet/view-all

http://homeguides.sfgate.com/troubleshoot-lowflow-toilet-flushing-47557.html

Make sure the black cover that comes up when you flush goes back down over the hole properly. If it doesn't, try adjusting it. If this doesn't work, you may have to replace it .

I once had a toilet that the chain to the flapper over the hole in the bottom of the tank had become kinked. This kept it from lowering the flapper all the way down. Sometimes the answer IS very simple! See if you can find what the jiggling fixes, then see if it is something you can fix more permanently. It is most likely the flapper, or something along the path between it and the handle you use to flush with.

We had this problem all the time from hard water. Calcuim buildup on the rod that lifts the plunger up. Take some CLR or vinager to the flushing mechasim. We finally got a water softener and no more problems

Sounds like the rod is bent. If you go to Lowes or Home Depot you can pick up a fluid master flushing mechanism to replace the defective one. First check where the rod goes through its mounting holes. there might be a calcium build up on it. Cleaning it may solve your problem.