Your continuing shortness of breath could be due to several things. The re-accumulation of fluid could certainly cause shortness of breath. The tumor could also cause shortness of breath. The effect of the radiation on the lungs could cause radiation pneumonitis and this can cause shortness of breath.
Each of these should be evaluated.
Recurrent malignant pleural effusion could also be managed by instilling a drug called 'bleomycin' into the pleura and that would adhesion of the two layers of pleura thereby obliterating the space between them. I don't know whether that option has yet been tried in your case.