MY hydrangea leaves are dying?

I bought this plant from a super market now finding out i shouldnt have well i noticed it was dying do i repotted into a bigger pot and started watering it more it was doing better but now its dying again well it seems like the leaves and the flowers are turning brown but there is alot of new growth and some of the branches at the root started turning woody and lots of new leaves are coming i know thats ok but what about my big leaves should i cut the leaves that are dying also i have it inside due to the fact its cold i live in an apartment so i wouldnt be able to plant it outside and what really sucks is that theres like no sun that comes in here to put it on the window sill i tried to do it anyway and it wilted i think it was to cold still?? so what should i do ?? please real answers i really dont want my lil plant to die