Got some therapy, journaled a bunch. I eventually realized having an abusive boss or working for an abusive company is the same as having an abusive partner. Recovery is similar.
As for the burnout, that takes time. You have to learn when to work and when to walk away at the end of the day.
In my experience, I realized I could spend a lifetime processing it and feeling bitter, or do my best to forget and let it go. It was helpful for me to change my environment and social circle, disconnect from past colleagues on social media, and change industries for a bit.
Things haven't been perfect for me, but I'm most back to where I used to be with effort and belief in myself (like 90% there, though I'm still more fragile if I make mistakes; though my current boss is particularly kind). It took a year to get mostly over it, though still unmotivated, and about a year and a half to feel recovered. To prevent it in the future, I also balance multiple projects, so my performance at work isn't as big of a deal to my identity anymore (which counterintuitively makes me better at my job).
1. I consciously decided to stop processing the past and let go. Instead, I moved on to the opportunities I currently have. I adjusted social media to remove reminders of my past workplace (including muting old colleagues). The questionable professional networking benefits wasn't worth the toll to my mindset.
2. Valuing myself a lot more wa helpful. I don't perpetuate the environment by by treating others unfairly for self-gain, but I avoid straining myself to help out others (where there's often no actual award or appreciation) unless it's mutual beneficial. I've found that a lot of people don't appreciate favours, and may even feel entitled to them from you.
3. Boundary-setting was important. This includes revealing less personal information at work (it can be used against me, though I'm trying not to take it too far) and declining overwork when I can (in a tactful, graceful way). This led to cutting off a one-sided friendship (where I was constantly asked for favours and support without receiving it back), because boundary-setting was challenged and not respected. It was painful, but ultimately helpful.
4. A shift in identity from my job to myself as a professional not tied to a workplace helped massively. I also started doing multiple projects, so negative problems at work don't affect my identity.
5. I kept up healthy habits. This includes a consistent sleep schedule, avoiding simple carbs when I can for diet, exercise, and avoiding outrage-producing information sources, like low-quality news websites and especially Reddit.
Due to additional time and focus, I started seeing someone in a fulfilling romantic relationship. I don't intend to reveal that past (I want to move on, as I'm a different person now). She's genuinely nice, too; she really enriches my life, though I'm avoiding repeating the mistake of tying my self-worth to anything external, including her. So, while perhaps I was the ideal, dedicated worker before, I'm still pretty good now, and happier with parts outside of my life.
In short: I committed to moving on and letting go of my negative past experience to reduce distractions. I then fixed personality flaws that tied my self-worth to others (by consequence, I learned boundary-setting, which counterintuitively improved my relationships). I also earned a belief in myself again, by sticking to hard but healthy habits and making good, consistent progress in developing skills I always wanted to have.
Last quick note: I got a lot of mileage from referring to "Feeling Good" by David Burns for self-treatment with CBT whenever I felt trapped by negative thoughts. "Intimate Connections" also by Burns helped me for relationship advice, which helps with enjoying the present.