Usually people with back pain have back pain due to muscle spasms/tightness/knots which are the back trying to protect itself from a percieved threat. The solition is to make the back stronger so it can handle more load, and train the mind to not percieve regular stressors as threats.

Hence, poor posture is not the cause of back pain, but an effect of the same source - lack of strength in the back musculature.

A heavy backpack should train you over time to be stronger to handle the load, so it is actually good for you.

And your sleeping posture exerts almost no load or pressure on the back at all, and should literally be irrelivant.

Actual solutions to back pain:

1. As another poster commented, get some magnesium. Magnesium is an electrolyte that allows your muscles to relax, and a magnesium deficiency can cause chronic muscle tightness and knots - which may manifest as back pain. Magnesium supplements are an option, or you could try just eating some spinach.
2. After decades of research, the science is clear - almost all chronic back pain is psychosomatic, or at least, can be resolved via psychosomatic therapies. Get a copy of the book The Way Out, and use the techniques described. It is available for free from the usual sources.
3. Get stronger. If you have significant, debilitating back pain, you should start under the supervision of a physio. However, if you just have occasional annoying tweaks, then you can start by just looking up some exercises online, going to a yoga class, starting lifting (with reasonable weights), or simply going for a daily walk. The stronger you get, the more resistant you will be to back pain in the future, all other things being equal.