Ok, so does it help at all to reframe it? To notice and acknowledge what it’s bringing up for you and how it’s making you feel?
Going out is going to make you feel tired and in pain, and afterwards you are going to recover by doing x/y/z? Maybe planning in something restful or restorative, some journaling, eating something delicious and simple that won’t trigger your ibs (weirdly thinking about a really delicious apple, some simple chicken soup or one of your safe foods).
Take a moment and if you are able think about two things, one - what’s in your control right now? Two - what or who can help you release some stress and sadness from your bucket? If you can consciously think about what you can control, maybe wearing something comfortable and bright and let go a tiny bit of what you can’t (you have to go out and get something done) it can help lessen how it makes you feel. For the second there is a good analogy I’m sure you know about a stress bucket. The ptsd will be gushing and filling it up, the pressure of the season and illness will also be adding to it. Then you have taps at the bottom to let out the water, some are bigger than others. Self compassion, gentle exercise or meditation, spending time with loved ones, doing gentle things you enjoy can all help to stop the bucket overflowing.