I agree with Flappys, try using a donut pillow. It really does help cut down on the amount of pressure and pain. If I happen to forget and not bring it with me, I'm done for the day. On a bad day if I have to sit for a prolonged period of time, I can go for about another 1 1/2 hours before I need to walk. Typical sitting tolerance is 30-45 minutes max at one sitting. Hope that helps.
Oh my work chair is a work in progress I got one of those coccyx cushions for like wheel chair ppl so I already got that - I even have an exercise ball at work cause gosh sitting on that is definitely softer than a chair lol (plus it counts as core exercising ;) ) but even that isn't enough (with applying heat thought the day) but once u can't sit... Nothing soft - not even soaking in the bath it's terrible u can avoid it besides laying on ur belly ...
Have u tried icing instead of heat? I noticed that ice actually helped me more than heat (even though the heat felt so soothing) if the problem is related to inflammation heat will make inflammation worse. Icing will calm the inflammation a little better.
I understand what it feels like to sit and it hurts all the time, my Dr's say it's coming from my spinal stenosis and 3 bulging disc's which generates down my lower back, buttock and down my leg. I to wish that there was a solution to my daily chronic pain. I sit on a donut pillow while driving, church etc...... I carry it with me when I know I have to sit for a long period of time!