Here a few things to consider that I've learned from those who have returned to a firm:
You can't bill hours to demonstrate your efforts and autonomy you once enjoyed as a revenue generator is gone.
If that doesn't seem appealing to you - really evaluate how pivoting in-house moves your career forward.
Associates tell me all the time, "I want to be on the business side." That doesn't always happen when you end up at some larger institutions. T14 legal education, Fulbright Scholarship, three years at a top five corporate practice and you're willing to throw that all away just to go in-house? You didn't work that hard to let that happen.
Lawyers who love to practice law often end up back at firms if this happens to them.
It might be the case that there's less incentives for your company to train you or to encourage your growth. If you want to be the driver of your own success, you need to find a place where clients rely entirely on your counsel.
I, of course, do not want to discourage attorneys overall from going in-house. For many attorneys it is the right move, and being the GC of a startup is a bad ass job! But, if you have gone in-house and you're starting to feel that buyer's remorse, here's the advise I'd leave you with:
Find an industry or vertical that you're really passionate about, find a firm/practice that specializes in that work and hope that you can make a move back to a firm and leverage your professional experience and be a proper advisor to your clients.