I think It is better if you take rest for now. I now it is extremely difficult for you to make a choice. You need to take into consideration about the pros and cons of your decision:
If you decide to go ahead with your program:
1) You will complete the program effectively only if you can participate in the group and give your best. You will have your rough moments given your condition.
2) As far as the school fees is concerned, I believe they can make an adjustment.
3) I don't know if flight fare is refundable but if it is, I think you should opt for it.
4) Regarding leave, if it is adjustable, you can try to do it.
If you are going to join the course anyway, just tell your school about your situation and I am sure they will definitely take your case in consideration and will put extra efforts to make sure you stay in the non-pain zone while enjoy your time at their yoga school.
Whatever it is you have to make sure you will be comfortable enough to enjoy your travel and the intensive month long course. And make sure to check out medical facilities in the area in case of emergency.
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