2010 Vauxhall Astra Battery

I made the stupidmistake of leaving my headlights on one night and it flattened the battery after a jump it seemed fine however after making the same mistake of leaving the headlights on again for only half an hour I'm having the same issues. I put a battery charger on the battery and it's telling me the battery is fully charged. And when I try to start the car it makes a clicking noise. What else could it be? I would be grateful for any answers so I can suggest them to the garage I take it too. Thank you