Ironically, what I find to be my biggest achievements are small but important things that I helped to happen; for example some experiences with students as a teacher. I recall one who was never too good at school but with me they ended up having top grades; I still remember their face when I told them they achieved a perfect score at a test. Another one was pretty scared from facing the course because they had a mental illness and their cognitive capabilities were diminished, yet we worked it out and they became a great student. A third case, an older person, told me by the end of the course that the experience motivated them to resume the college degree they left unfinished long ago. Stuff like that.
I'm not the best teacher out there, don't consider myself to be so at all, but these little achievements are an amazing feeling.