I can't help you....I'm in awe that you have kept one for several years! Do you fertilize it some to make up for depletion after the bloom? That's just a guess. Also, mine have done best when they have not gotten too much water. I'll look forward to other suggestions ...c

I have several that are blooming now for the fourth time....they were Thanksgiving, Christmas, Easter and now just Spring cactus, I suppose. I have had them in excess of fifteen years and keep them a little root bound. I do fertilize three or four times a year and never let them dry out completely. I hope you find your problem as they are wonderful little plants

I have two young christmas cactus and one of them is also struggling after the blooming period. I'm not quite sure what the problem is, but, in my case, I think it is in too much sun. The other plant that is in partial shade looks fine, so perhaps it's the location? I would also check to see if perhaps the soil has become depleted and compacted.
Molly

I bring mine inside for the winter, just as it sets buds but by the time I bring it back outside in the spring, it also looks very bad and is literally covered in aphids. It's the only plant that I bring in that does that during the winter. I take a paper towel dipped in soapy water and have to scrub off the aphids. then is finally recuperates.

After reading these posts, I should tell you that all mine are in the house in bright light...........not sun. Perhaps that is the problem........way too much sun. I have one in a hanging basket by the sink and it gets only the light from the flourescent light. Maybe moving them to a bright location out of the sun would help. Hope so as I love these little plants

I repotted it this weekend and the soil looked okay, not too wet or too dry. But I threw it all away just in case there is a fungus in it or something and used fresh soil. The roots had not spread out into this pot at all even though I think it's been in this pot for a couple years. I washed off as much soil from the roots as I could.
I do have it outside in partial shade but will make a point of moving it to deep shade. I know if I brought it in then my cats would start digging in the soil all the time. Thanks everyone! :)

I think your partial shade is fine. When mine is outside (March through November), it's on my east facing porch and get full sun for at least 1-2 hours and then very bright shade after that. And if you do bring it in, you can put egg sized rocks around the base of the plant, that keeps my cat from digging. I do that with all the plants I bring in to the house.