My last game that I lost, I feel like at fault because of how I played support during teamfights. Both teams were even and the game was at the balance, the final teamfight (as nunu) I completely rushed and exhausted/snowballed the enemy AD carry leaving mine's vulnerable.. was it a wise choice to do that and was it completely my fault and what should I do next time?

As a support, your job is to protect/peel for your ADC most of the time. As nunu, you're incredibly good at peeling with your snowball and your ulti. basically, if you see people trying to go towards your ADC just snowball/bloodboil your adc and ult. It kind of depends on what the other players on your team are doing.

support need skills