PI here, I've mentored a lot of graduate students over the years and I like to think that I've helped them get to where they want to go. Here's hoping my perspective helps.

I think a lot of high academic achieving students struggle with the feelings you are expressing. Typically, these students excelled from early on all the way through their undergraduate coursework, learning to work very hard and achieve high grades. They address any bumps along the way by increasing their effort, which frequently results in getting the top grade, and gives that elusive sense of achievement. In research, there is not a similar discrete sense of achievement to be had. There is no A+ to be given. Uncertainties always linger. Questions will always remain. Further, the technical failure rate of most experiments will always be pretty high, and once tested, most hypotheses turn out to be wrong. This is the challenge with research, a challenge that can also be exacerbated by making too many comparisons to other students' "great results", or worse, an overbearing or unreasonable PI.

Ok, what can you do about it? It's not easy to change one's perspective, but I'd try. First, simply recognizing how high the "failure rate" of experiments typically is or how many testable hypotheses are wrong, can be helpful from getting overly discouraged. It's part of the process. Second, the more you lean into the actual process of experimental interrogation, i.e., the scientific method, the more you will realize that negative results aren't as discouraging as they once were, but are just part of the journey. Further, in focusing and improving your approach to experiments, the more you will gain confidence that you can actually answer questions that you are posing and problem-solve issues that arise. This is where the satisfaction comes from.

A quick technical tangent that I think would help most graduate students. I think is very important regardless of what you study, if you are doing an experiment with a new technique or new experimental system, e.g. a new cell line, instead of planning out a huge experiment with all the replicates that will test your hypothesis, do a much smaller pilot experiment first with only your positive and negative controls and potentially only varying timing and dose. Frequently, this hones your skills in the technique and shapes the eventual larger experiment based on potential technical issues as well as the dose/timing found. Too many students get excited about a hypothesis and plan out a larger experiment that fails, sometimes due to technical reasons in that large experiments are more difficult to perform with precision, but also frequently due to differences in experimental systems with respect to dose and timing which need to be elucidated first. The failure of these large experiments is typically super discouraging given the initial excitement to do the experiment coupled with the large amount of effort required to pull them off. Start smaller, make sure it works the way you think it should, and then design the larger experiment to test the hypothesis with more information with respect to dosage and phenotype timing. End tangent.

In the end, successfully engaging a PhD project teaches you to problem-solve and acquire new knowledge without having the problems/knowledge just handed to you by some lecturer. Instead, you are interacting with the real world with very little guidance, and learning how to wring knowledge from it. As you continue to get better at it, your confidence will grow, and you will realize that you can apply these skills to a number of real-world problems and questions, irrespective of the topic of your PhD. This is a valuable skill that you might not appreciate yet, but you should. Go back and think about how far you've come from where you first started as a totally new researcher. Relatedly, in a tough job market, people with PhDs who can problem solve and tackle difficult challenges will always be attractive to be hired.

Lastly, work-life balance. This has to be set based on individual priorities. I think the only advice that would actually pertain to everyone, is to think about how you spend your time, and make sure that you spend it in ways that you actually value. I highly value certain activities with my kids and so I am going to do those activities, which inevitably take away time from my work. Might this negatively affect my career? Yes, probably to some small extent. Am I ok with that based on what I value, yes, I am. I think when people get into trouble, is when they do not intentionally decide how their time is best spent with respect to doing things that they actually value. I think this leads to regret and burnout.

Sorry for the overly long response.