Can I resign as a Corporal?

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.

The Army either needs to go the way of the USMC and make all E-4s NCOs or they need to get rid of the CPL rank and make all E-4s SPCs. I’ve seen a lot of people who met all the requirements and were plenty qualified to make SGT but had CPL slapped on them instead.

I feel your pain, but if you’re that close to being promoted then I’d just buckle down and tough it out. If you tried to resign it then your CoC would probably view it like a number of people here have, as you not being ready for your 5.