Shifter moves all the way to high gear by itself

[juxpin]

Rear Derailleur shifter: Falcon Index System thumb shifter.

The high and low limits are set correctly because I can shift through all the gear with no problem. I just have to hold the shifter in place. If I let go, it will move all the way to high gear (small cog). It won't stay in anything but the highest gear by itself.

[GeorgeET]

Your shifter is broken, or loose. No idea what type it is or on what bike.

[DaveM]

If there is a screw on the top of the shifter, right at the center pivot point, you might try tightening that up a bit. On some shifters, this screw needs to be tight enough to keep the shifter from slipping to high gear, but not so tight that you can't move it. But a picture of the shifter would help us diagnose better.