I'm a few years older than you and have very bad IBS under gastro care (being investigated for additional Crohns etc...), arthritis and meniere's disease.
I have a very demanding job but try to do the best I can to get my symptoms under control or at least manageable so I don't get this problem.
Could you pick a more suitable job to your problems and do anything more about the IBS and arthitis such as medications, other therapy and lifestyle changes?
What is the main reason that the conditions keep you off work?
Have you been properly investigated in light of your tummy problems as well to rule out anything else which could be causing it?
Stress can be a major factor. My tummy and joint pain has been evil the past few weeks as I had a relationship breakup, had to move etc... now I'd settled in my lovely new place and with the help of my osteopath, it's settled down a lot and is a lot more manageable.