Ducher Rose withy Black Spot

[Vespertino]

I bought a really nice little ducher rose in February, and it gave me some pretty blooms for a couple of months... Then the aphids attacked, coated all of the new growth and bit the plant to shreds. As I got the aphid infestation under control some damp/wet weather rolled in and I started to notice some yellowing leaves. Being a novice I though it was damage from the aphids and let it be, and just tried to take care of the plant by killing any aphid on sight and giving it some plant food. Leaves and rose buds attacked by the aphids were were growing in crooked and I thought the leaf discoloration was related. More and more leaves turned yellow with spots and started to drop, and after some research I realized I have a case of black spot.

I'm a little confused on how the black spot became so bad: my insecticidal oil spray also doubles as a fungicide and I'd been spraying the leaves regularly.

So I've pruned off any yellowing leaves, and now my poor rose looks ratty and pathetic- half of the leaves are lost. I've given some thought to re-potting it (its still in it's original container from the greenhouse) in clean soil but I don't know if it would make a difference.

Is there any hope for a rose badly infected with black spot? Or have I already done all I can do for a doomed plant?