I have suffered similar problems for years, and got similar results. The best help I received was from a physiotherapist at the local hospital via my GP.
It seems mine is mostly postural, so with a bolster behind my back in my car seat and care not to sit slouched I can improve things.
A great improvement happened a few years ago when I changed my car. I hadn't realised how the drivers seat of the old one was putting my spine in the wrong position. Also some of my worst episodes happened when my children were babies, due to slouching low on the settee holding a bottle and baby for periods
You must always ensure that your lower back is pushed inwards so that your spine forms an S shape, which is where the bolster comes in. They can also show you exercises to strengthen the muscles and inprove your posture. Good luck.