Dear friend,
I don't know if you have them already, but look for Chopin Etudes op. 25 revised by Alfred Cortot. There you can find lots of exercises he recommends for the different etudes. In particular, you have to look for the Etude op. 25 # 6 (thirds). He presents a lot of different exercises you could apply to your left hand, and the passage will then go smooth. Anyway, don't stick to one passage, if the work doesn't go, leave it for a while, study some other difficulties and works, then come back to it, and it will go better.
Good luck.