How much of the MEPS process do I have to complete again after having withheld some of my medical history during my first attempt?

You need to speak to a recruiter but as you gave up, fairly certain you have to start the process all over again.

Pretty sure you have to start over. Regardless of how much time has gone by and you thinking that your medical situation, etc has not changed MEPS will not concur.

Depends on two things: How long it’s been and how much of your physical they completed.
You won’t need to retake the ASVAB as long as your current score is from within the last few years.
It is HIGHLY LIKELY your physical was not completed, meaning that you will have to entirely retake your physical, but in very rare cases I have seen applicants allowed to complete their physical and get held up pending documents. Those applicants still had to return to MEPS but were only subject to evaluation for the condition in their documents. Again, timeframe matters, because I had those applicants back on deck within a week because they didn’t flake on me.