There's a lot you can do to help yourself.
Get a long handled fork, and don't take big forkfuls especially of wet bedding. Look at your technique - try to avoid twisting your back too much & make sure sideways movement is more from the hips & legs. Use your tummy muscles to support your back. And I may sound like a granny but keep your back covered and warm while you work!
Do they have to have nets? If they do then give several small nets rather than one big one, lift them properly (no swinging over your shoulder - never, ever, ever!) & use a barrow to move them from the hay barn to the boxes even if it's only a short distance. I find small bale hay is far easier for filling nets so maybe that's something to look into.
Water buckets are better half filled & carried one in each hand so you're balanced. If you have large trugs for water in the stable then don't overfill them because you'll just have to carry it out the next day & put the empty trug in the stable & take water to it with normal buckets.
Avoid bending to lift stuff, instead crouch down & when you pick things up keep them close to your body again using your tummy muscles to support your back.
Look up how to lift properly & always do it, even for things that don't seem heavy. Even when you aren't at the yard keep an eye on your posture. Depending on what you've done gentle sppwimming or walking can help keep it mobile & strengthen it up, and if it's a long term problem maybe consider some yoga or pilates classes.