Congratulations on completing the retreat. I'm going on my first Goenka retreat early December.
I've had similar responses after retreat. Sounds like you might want to back off from the insight practice for a bit, and just do concentration. Anapana sounds like a good approach, noting of the 3 C's is likely to just be pushing on a string at the moment. I read that the Goenka retreats end with
metta practice, which would probably help with the despondence. Volunteer work on something people really appreciate helps, too.