Is there another way to move your right foot onto the small sharp foothold so you can get your left on the good hold ?
I had a similar 'mare this afternoon, couldn't get the heel to sit on the little edge at all.
There is a small sharp smeary thing right on the very edge of the arete, equidistant between the edge and the good bit you're aiming to get your left on, I had some success stepping up RF on this. I'll persevere with it once my finger's regained sensation from holding the pebble.