Any recourse against my employer who is forcing me to choose between my health and my job?

I was out of state for a business meeting and asked to leave the final meeting early to enable me to catch the first of only two flights home that day. Leaving 40 minutes early would save me 6 hours. I have autoimmune disease and travel is difficult at times. Avoiding 6 hours in an airport can be imperative. I was told no, despite the fact that my two coworkers (there are only 3 on my team) were already gone, getting permission to leave a FULL DAY early. I was told to purchase the 2nd flight and did and then I was released from the meeting a full hour early anyway so I could have made the 1st flight. I changed my ticket anyway and ate the cost. Since then my employer has decided to call all travel essential (it's not) with total disregard for my health. I am an 18 year employee and for 16 of them my management was empathetic and accommodating. My new mgr is mad that I raised a fuss two years ago and now has basically said I will be fired if I cannot do everything he orders me to do, even if my only two peers (identical jobs) are excused. I feel ganged up on. Harassed. I have never missed an essential trip but twice in 18 years I was unable to travel to a meeting or seminar and my management was so compassionate. How am I supposed to go to work with the knowledge that, if I have a flare up, I can be fired for it? How is this okay?