Hezbollah mainly joined the fight (only recently) because of terrorists displacing and murdering Shiites and Lebanese living in Syria, by the border. If they didnt join the fight, they would have been next. Best defense is offence.
The tide turned because something changed in mentality in Syrian Army. There are some reports that Iran sent about 150 IRGC generals last year to Syria, to not only train Syrian soldiers in assymetrical warfare, but to motivate them. Hard to judge the validity of these reports.
But even the terrorists in Al-Qusayr admitted that they experienced a swift change in battle field. Something had changed in Syrian Army's mentality and that they were more motivated than ever, to crush the terrorists at all costs.
We all have to remember that the Syrian Army is conventional force, trained to engage in conventional war against state actors. And initially not trained to fight in this guerilla set-up