How do I prune and care for my potted double gardenia?

I have a double gardenia plant in a very large pot outside. Bloomed massively this first year, but after blooming most leaves went yellow/dropped off.Plant itself hasn't grown - branches/stems the same as when I bought it.I'm in Santa Monica, CA 90403, right on the coast.Gave it acidic plant food till October.Now the end of December & it should be pruned, but I don't know how far back - or at all? Add peat moss and/or mulch now after pruning? Please advise ASAP! Too gorgeous to lose...thank you!