I left a legal career of 14 years recently. I was in my early 20's when I started. At the time my boss had only two employees, one of them was me. I stuck with my boss and watched her business flourish.
When I decided to leave my boss immediately disowned and blocked me. I wasn't able to carry out 2 weeks to say good bye to anyone, no good bye party, no well wishes. She was so mad I left she wouldn't give me a letter of reference. I was essentially erased.
For the last 6 months I have been depressed. It was the realization that I spent 14 years thinking I mattered when I didn't. I felt like I wasted a huge chunk of my life helping someone else get ahead. It was a crushing realization.
My words of advice are don't go above and beyond for your job, keep work separate, your colleagues are not your friends and HR is there for the employer not the employee.