Heya I had a similar problem - mine is 16 months and sometimes I still rock 😭 it’s been the worst part of having a baby for me. Only thing that saved my back a bit was trying a different hold each evening (as she got bigger she preferred head on my shoulder which was a godsend), using an exercise ball and having feet elevated on a block to relieve back pain. Do you still breastfeed? I’ve tried to stop rocking altogether and just wait til I’m sure she’s really tired, sure that she wants milk and BF til she falls asleep. It takes effing ages but it beats the hour OR MORE of dreaded rocking
Is your partner available to help? My husband is having back pain, and he rocks our daughter to sleep every night. A couple of times I have gone up to transfer her so he wouldn't have to lean over the crib. So he does the rocking / putting her to sleep part, but doesn't have to destroy his back putting her down. Barring that, at 13 months if you're comfortable with it, it is not unreasonable to consider some sleep training. She can understand you when you say it's night night time, you could try putting her in her crib to sleep. Despite still rocking ours in the evening, I do this now when she wakes up at night, and it's maybe 10 minutes of crying, then she'll go down again. At 13 months, trust should be well established and you aren't going to destroy it with some gentle sleep training. Especially to save your back.