It seems obvious that you are simply carrying to much weight (This is an ultralight hiking forum after all, so if you are carrying enough weight to hurt your back then you shouldn't really be on here!)...So how to lighten your load? First you should start with the
backpack: Most backpacks are only "
water resistant", not "
waterproof"...A lightweight waterproof backpack of a given empty weight will actually work out lighter than a lightweight water resistant backpack of the same empty weight, because with the latter you have to cover it with a rain cover when it rains, which adds extra weight, and many don't trust the rain cover alone and also use dry bags inside as well, which adds even more weight. So to save weight, look for backpack that is guaranteed to be waterproof. Personally, I went for the Mountain Hardware Scrambler RT35 Outdry...Under 800g empty, 100% waterproof...You can even go swimming whilst wearing it and everything inside will stay perfectly dry!
Backpack size: It is not essential to carry all your items inside your backpack...Most backpacks have "daisy chain" webbing provided on their back to allow the user to tie bulky items on the outside, saving space on the inside for smaller, easier to pack items. This can also enable you to downsize your backpack, which in turn can reduce the empty weight...You do not need carry a huge 50 plus litre backpack when a 35 litre backpack, with some items stored externally, would suffice.
Items you carry: Needless to say, the less items you carry, the lighter your backpack will be, so sit down and think really hard at what items are actually essential for your hike and what can be left at home.
Swap for lighter kit: Simply replacing heavier items you normally carry with lighter equivalents that do the same job, can save a lot of weight and can often save a lot of space in your backpack too. For instance, replace your sleeping bag with an ultralight Down filled version, and you can save hundreds of grams and they can be packed down really small in an ultralight compression sack. Of course replacing all or most of your kit can be expensive to do, but what is more important?...Much more comfort and having no back pain, or a lighter wallet? You will have to decide that one.