Let a woman I loved go. Long story, but it wasn't going to work out, plus the longer it would have lasted, the more people would've got hurt. I'm glad I found love again years later, this time with no strings attached.
Other things that may or may not be "the hardest things I've done", but still things I'm very proud of:
-Traveling 4200 km in a car over the course of six days during last summer, when me and three friends took a road trip to northern Norway, driving across Finland twice. There were four drivers, so no-one had to drive any more than they humanly could, but it was still a long journey. Fun as hell though, and I would happily do the same this year.
-What I consider my life work and purpose here: nearly 150 individual made songs over the course of about ten years + other random stuff on top of it, such as different kind of videos, that each define my life somehow, big or small. All that stuff shape the life I've lived, and what I've encountered during those years. Speaking of terms of time, this project is then probably the hardest thing I've done, since I've devoted so much time to it.
-Having existed on this planet for 27 years and finding at least some joy in each day, which in the current world isn't always something that could be taken for granted.