How to fix a 9 spd Shimano 105 sti?

Hey, sorry I don't know all the technical terms.

On a ride this past weekend my right shifter stopped down shifting. I can up shift, but when i try downshifting the whole lever moves. After a few frustrating miles I realized that I can downshift if I hold the main part of the lever straight with two fingers.
Does anyone have an idea of how to fix this, or If I can keep riding with this problem for a bit until I can locate new ones?

Thank you in advance!

Common problem with these levers. You can try to use a "STI repair kit", Flush with WD-40, followed by a spray lube. 'Sometimes' can free these up but the 'big lever traveling with small lever when trying to shift to a higher gear' is the failure mode of these. Holing the big lever outboard when shifting the small lever will make it work but tough on the bike.No way to 'fix', ala Campagnolo. Cannot really take apart and reassemble, no small parts available from shimano. I recommend replacement.