POOR The dragon fruit cactus aka pitaya

THIS POOR PITAYA CACTUS needs a lot of help. I'm going to have to do a lot of reading. My wife told me to cut it so that it wouldn't grow so tall & it continued to grow taller just splitting into 3 shoots instead of one. I have to read up on pruning, watering
supporting and fertilizing. We lost 6 immature fruits (maybe more) this year