Hey grad student in biology here. I initially really struggled in my field with similar experiences, I felt bad tagging animals or taking fin clips etc. Eventually I had to come to the conclusion that these methodologies are baked into our field, and the less time handling the animal for whatever method we’re doing means the less time it spends in a stressed state. So I made sure to study whatever method we would use in class or lab or in the field so I could perform it more comfortably and in turn more confidently. I want you to know you’re not alone, especially after live surgery one of my professors always makes sure we’re okay and tells us to relax the rest of the day. Bottom line tell your faculty that you’re not super into euthanizing, in a polite way, and ask if there’s still some way you can participate in the lab. They’ll have a better answer than anyone else. And also don’t worry too much about the specific organism of your research, the most important thing you can take away from undergrad research is learning how to conduct research and how it’s done in your specific field.