In my experience, the sacroiliac joint (SIJ) plays a similar role in cycling as disc or vertebral joint dysfunctions. All of them can lead to pain-induced relieving postures that make prolonged pain-free cycling training almost impossible. At the same time, however, a "crooked seat" can also be caused by other factors such as a difference in leg length, muscular imbalance, but also technical errors - to name just a few. In winter in particular, you can practise your round pedalling technique on the roller in front of a mirror. Riding with one leg on the pedal is also an important exercise.
You should correct imbalances by training the respective muscular counterpart. Targeted physiotherapy may also be able to help you. If you have legs of different lengths, you should try to compensate for this by looking at the shoe between the sole and the clipless pedal. Pain in the lumbar vertebrae/pelvis transition is often caused by a weakness in the muscular chain foot-spine-hand. You should therefore train this region in winter. If you are lucky, you will be able to cycle again in spring without pain.
