No special expertise here, just best wishes. Some experience with personal growth, though - shifts in perspective, different life situations over time, etc (50something now, thin FIR, several previous career shifts).
Suggestions/ideas/leading questions:
1. Which do you prefer in your gut - retiring, continuing your work, or continuing to work but changing something about it?
2. If retiring, don't retrain unless you can prove on paper than that you will reach FIRE sooner.
3. If continuing, focus concretely on reconciling with current situation, learning to be happy now. Do the therapy, begin a gratitude journal, etc.
4. If changing work details for emotional fulfillment, seek to change the work details without retraining if you can. As appropriate, change your work habits, modify job terms, search for job that meets your desires more precisely, etc. Perhaps first, just shift focus on the job to work relationships instead of work accomplishments (bonus - no external changes needed, implementation cost zero).
5. If the higher-wages-in-future money pulls you emotionally but you can't prove on paper that there's a payoff before FIRE, don't retrain yet. Dig deep into the reasons. Again, therapy. Also do any items from 3 and 4 that have a chance at affecting your emotions. If unsure, do all of them!
:)
I applaud your question. It shows insight. You've established a path to FIRE, likely solving the financial equations of your life. Now you're addressing the next step of growth, happiness, by offering up the most important and confusing question in that area you have. Keep banging on it. You can do this too.