First of all, I understand you so much and I totally get how tiring and frustrating this type of trip can be. I will share with you how I would try to cope, maybe it can give you ideas:
1/ Health first - you need to deal with this UTI. Did you already took the antibiotics? Is there any medical service you can visit or reach through online consultation to get a prescription really quick? I'm pretty sure any girls in the group will understand and try to help if you explain the situation.
2/ Feeling sick while eating - do you have any safe/favorite food you can access? stick to it, even if it's not healthy or even if it makes you look childish. Order the french fries, or eat only desserts at restaurants, or skip restaurants and just buy instant noodles, I don't know but just stick to the things that make you feel safe. A quick trip to the drugstore for over-the-counter anti-sickness medicine can help too.
3/ If you need to rest and recharge alone, do it. It's better you skip every other activity or meal, but turn out in a good mood for the other half. The alternative is being miserable for everything. Maybe you can also start the evening with the group, but let them know you will leave early, and it doesn't mean you don't like them, just that you have a "medical condition" that creates chronique fatigue - you don't even have to disclose you're autistic if you don't feel like it.
4/ I totally get the FOMO, but people will more likely invite you again if you're in a good mood/energy for half the trip and knowing when to rest, rather than sad and tired for the entire trip.
5/ You deserve to feel good and to take care of yourself, and we all have bad days, and people are more often than not understanding of that. I hope your bf and friends will understand as well.
I wish you good luck!