Backstory: When I was ~21 I was in an accident and after that had some remaining health problems and got depressed. This led me to in effect drop out of university and not do anything productive until I was 25 (this was in europe and I lived on 'on sick-leave student assistance' and an inheritance).

At 25: I had just gotten a part-time job at the university (which I managed since my depression was being dealt with, and some underlying medical issues had been diagnosed and started treatment) and was aiming at working there while catching up with my degree. The job was semi-related to the degree. In practice I made myself irreplaceable at the job and moved up to full-time and getting fairly consistent promotions (my effective salary has been raised by on average ~60%/year since I started working there), while ignoring my studies for my degree (but taking some courses in my new field).

Current status (early 30s): I have effectively topped out at my university position unless I move into management / political (getting funding) positions, which I don't want to do. Also I have grown tired of the limits of academia (and my department) and feel that I need to move into the industry. I recently failed some interviews with some FAAMG companies (due to not taking taking preparations seriously enough, and not thinking that I actually had a chance of getting the positions even though I moved onto later-stage interviews). I just signed a contract for working at a company in the financial sector and am currently doing the hand-off at my current job.

Tips for myself at 25:

I will give advice specific to me as well as a more generic version that applies to anyone at that age (or, probably, any age).

- Be more honest with yourself and realize that you will not finish your degree, and put more effort towards your work. Generically: Reflect on your (career) goals and how you will realistically fulfill them. From this build objectives that are not redundant or useless. Reevaluate your goals periodically.

- Be clear to your boss about what your plans for the future are, since he is in the best position to help you with your career development[1]. Generically: Don't be afraid of accepting help from people who offer to help. Your boss is supposed to be your advocate against management, and to help you reach your goals within the constraints of your current company/position/... (this is dependent on your boss being a reasonable/good person/manager).

- Be less passive in fixing your problems outside of work. Generically: Strive for having a good balance between your work life and your personal life. Either will affect the other, both your performance at work as well as your interpersonal relationships.

- Take your health problems seriously, get all of them checked out. Generically: You are your only health advocate. Any health professional you meet will be overworked and have a big incentive to dismiss your problems as not relevant/serious. They also will have their pet issues that they will try to fit you into. Investigate your symptoms online if you can be somewhat objective about yourself, and know how to read lists of symptoms.

ETA: Thank you for asking a great question by the way. It has gotten me to reflect on my last couple of years in a very productive way.

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[1] - this only applies if you have a good boss, YMMV