I was medically retired and I feel there's post service things I have forgotten to do. What is the recommended best course of action?

If I am not mistaken you should still have access to Soldier for Life. Is there something that you need assistance with? I know it may be challenging to get out of the Army. I got out my first time when I was 22. It took me a while to figure it out but after a few months I found myself. sometimes we aren't able to identify as a civilian. But you need to make that transition. It seems that you were forced quickly. But places like this can aid in pointing you in the right direction. I would also look at Vet organizations in your area.

I don't have specific information that can help but you have taken a step by asking here for assistance. Hopefully, you will find the sources and information you need. Hope what ever the medical reason for your discharge was is under control. I pray that you have success in all things.

SGT Charlie L. and Capt Gregory Prickett plus others have excellent advice about finding a Veterans Service Office to assist you with the VA, state, and local veterans organizations. Here's what I'd tell you if your were my Grandchild (I have grandchildren about your age).
-Get a job. It looks like you have a job with Amazon. I salute that. You could apply for civil service jobs (http://www.usajobs.gov). There are advantages in the civil service system for veterans and especially for disabled veterans. It may take a while to get into civil service, but it can be a lifetime of gainful employment and good retirement.
-Add to your training and education. The Army started you off with training in food service. Capitalize on this training if you generally liked food service. Use your VA benefits to add to your culinary skills and knowledge. Community Colleges often offer training in culinary arts for a reasonable price. Beyond that there's a four-year degree in hospitality management or hotel and restaurant management.
-Find a Church. Visit as many as you need to find a Church you're reasonably comfortable with. Join a small group in that church that seems to match your age group or interests. Volunteer. Use your skills and knowledge to work through the Church to help less fortunate people. BTW if you aren't Christian, then seek out a religious institution consistent with your religion (synagogue, temple, mosque). If you have no religious preference or experience, then try the Church first.
-Don't become your disability. I don't know if your medical retirement happened because of a debilitating injury. No matter. If you have a disability, take full advantage of all the programs the VA, state, and local offer. Don't be just the person with a brace, or artificial leg, or wheel chair. Continue to be you and bring the disability along as an afterthought. Do some web research and learn about "reasonable accommodation" and the Americans with Disabilities Act (ADA).