The only thing we can suggest is changing cars for something as practical but with better seats. The problem with van-based MPVs such as the Doblo is that the seats don't tend to be very supportive. In terms of changing them, you could try other Fiat seats but you'd have to check the fitment. Changing the front two could be a possibility, but you'd need to get a competent mechanic to carry out the work as there are obvious safety implications.