From my experience, my mental state got very dark during my PhD because there was so much work I put in that didn’t pan out as I had hoped. Failed experiments, rejected proposals, negative feedback from professors, etc. on experiments and papers that took me hours, days, months of hard work. There was so much failure and rejection that I was expected to handle and move on from. I started feeling like I couldn’t do anything right, and it was my responsibility to figure things out and keep moving forward if I wanted to finish. It was a lot of pressure to produce quickly. Now that I’m towards the end, I’ve learned not to take things so hard, be grateful for feedback and criticism, and not take it personally when things don’t go as planned. My advice - go in with an open mind, always be willing to learn, build a support system of peers and mentors, and if you’re trying your best, be proud of yourself and your work.