Can I resign as a Corporal?

You can always go down in rank, but remember, when you're looking for E5 it will be remembered that you didn't want the "responsibility". This is a chance to make yourself the obvious choice for SGT, but if you don't see it that way it's also a chance to stymie your career.

If you are not being given the respect that goes with the rank, you should speak to the 1SG. If your unit saw the potential, and you are subsequently producing they should give you that respect. Are you earning that respect?

Have you separated yourself from the junior Soldiers, do you disseminate orders and guidance in a manner that earns you the respect of being and NCO? Are you still carrying yourself in the manner for which you were selected to be a Cpl. while I agree. That it can be thankless, it does not have to be. Lateral or not, you are an NCO.

Much like the 1SG, you were selected amongst your peers to lead Soldiers. This indicates that your 1SG has what it takes to lead Soldier over other unit E8’s, and you stringer than the other E4’s. Let the leaders that selected you guide you through this. Thank you for your service.

The process is to get promoted.
Corporal is definitely a thankless rank. I was one in the National Guard and when it was offered to me on Active Duty I turned it down. In my unit being a corporal meant you were treated like a specialist until there was a detail that required an NCO.

CPL is just a lateral promotion, like a 1SG. It can be bestowed or revoked by the commander at any time.