Try hardy fuchsia,s, azalea, hebe, acers , hellebores will do well as will anemone,s . I have the same problem in my garden and these are things I have managed to keep.

Couple of things - Hydrangea, if bought in flower, may only continue to be in flower for another couple of weeks, depending when you buy and plant it. Fuchsia should more or less continue to flower from July onwards, with slight pauses sometimes.
If your garden is in deep shade, completely shaded by trees, then you will need to look at shade loving plants - if hydrangea and fuchsia are not successful, then the less colourful things like Aucuba, Sarcococca, ferns, etc. However, don't really have enough info to go on to comment any more than that - it may be that you're not watering sufficiently, for instance, after planting, or that the soil hasn't been prepared properly prior to planting, or the soil is poor and not particularly fertile. Without more info, hard to say more.