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).

Do you actually have all of your authorized awards on your record? I've met more than a few Soldiers that don't when they got out. Are you fully out? Do you still have access to JAG? If so did you get a free will or power of attorney if needed? Once you're out it costs you money.

If you're having trouble adjusting consider looking into local veteran organizations.

The only thing you missed being rushed out is your TAP classes, which are all voluntary. You're authorized to go back to any of their workshops that you would like to use. You VA medical benefits were already submitted as part of your MEB. Now you just need to figure out what you want to do with your life.

I got out of the Army at 21. I didn't have any dependents so I traveled a bit doing seasonal work, snowboarding and working at a ski resort in the winter, working in Alaska in the summer. You have a little bit of permanent income, you don't have to rush to using your GI Bill yet if you don't know what you'd like to do. I was a cook when I separated the first time. The best part about being a cook is you can travel anywhere in the world and find a job immediately.

Don't worry about what you need to do to be a veteran, you'll meet more veterans in your travel, it's inevitable.

I recommend going to a local VFW or DAV and ask to talk to one of their Veteran Service Officers. They can definitely help you with any VA issues and probably answer many of your other questions. Best of luck!

When you say there are things you have forgotten to do, do you mean specific tasks or more generally how to be/act like a "civilian" again?

Unrelated: You could be a poster child about why transition in the military is so difficult.

Make sure you are signed up with the VHA medical center closest to you. Literally just walk in with your 214 and ask how to get an appointment. Get assigned a primary care doctor. Get on the Internet and get signed up for a username and password for the VA as that is where a lot of your information will be located. You are entitled to a VA home loan and you ca get your certificate of eligibility from there.

If you have questions about your GI bill you can call most colleges and universities and they will have a veteran resource officer or employee that handles that paperwork for you.

As others have said get with your local “vet center” this is a state agency not the federal agency. Each state has a list of additional benefits for veterans. Google what benefits am I entitled to as a veteran in _______ state. VFW, Disabled American Veterans, American Legion, and all the other veteran resource organizations have tons of information and also are a great way to network with like minded people.

You will have veteran’s preference on federal jobs, state and local government jobs in most instances. There are tons of reasons to be proud of your service and a lot of good people are there waiting to help you.

outprocessing is just a series of briefs mainly on two things. Getting an education or getting a job. The Marine Corps also had an emphasis on starting your own business but the focus was either work or school. Med Board briefs focus on getting you the tools you need to better navigate dealing with the VA. If you are medically retired then you should have a blue retiree card and also be signed up for tricare medical care. If you are not there then that would be a start. If you didn't drop off your medboard findings to the VA to get your disability payments started then do that. If you have injuries not covered in your disability claim work with a veteran advocate to submit a claim. Check your State and see what is provided to veterans with your disability percentage. Look up local veteran non profits and get involved if that sounds like something you would like.

That is what the Veterans Administration in for. Contact them and get a rating. They may pay you more.