I am in the process of getting my paperwork in order this year because I don't trust my spousal unit to abide by my wishes if anything happens to me, particularly if I die by suicide. Also, I had controlling, invasive parents and understand not wanting anyone to ever interfere in your final decisions.
Please excuse the wall of text that follows.
I worked in healthcare for a number of years, I did some time in EMS and have worked for a few different services in different jurisdictions. Though I have little practical experience with patients having DNRs available during the call - I only ever encountered a handful during my time in service - our training and the policies of the services I worked for told us that if there is a VALID DNR present at the scene we do not attempt to resus, even if the patient's family is begging us to. If there is no valid DNR then we must attempt resus. I have responded to suicides, but none of them had a DNR and were often showing obvious signs of death so we didn't always work the body. One of the few things we were told about responding to suicides and attempts (not much was said, mind!) is that we don't get to determine whether somebody was "of sound mind" because we were not in a position to determine that, it was not something we were trained to do, we had to take the scene as it came and work the patient the way we would any other. I never had a patient who came in as an attempt so I can't speak to how we handle them if they are conscious, and we would probably contact our medical control in that situation. As for doctors ignoring DNRs on a case-by-case basis, I've never heard of that happening when I worked in either ambulance or hospital service; all of the doctors and nursing staff I worked with considered the DNR inviolable, so it may be worth asking a lawyer.
Please bear in mind that this is just MY experience with the services I was with some time ago and in the areas I was serving, other people are likely to have different experiences and knowledge. Every service is different, every jurisdiction is different, and things can vary wildly in some cases. Your mileage may vary.
Advance Directives are legally binding, if you have specific wishes regarding your care and body disposition then you need to fill out and REGISTER your Advance Directives. They are typically free, can usually be filled out and registered online, and will be available to hospital personnel via the registry. The only thing you really can't do on your own is the DNR, which requires a doctor to sign off. You will want to print out a copy and keep all of it together in a safe place after you register it, and it would be a really good idea to put a card in your wallet and/or information on your phone for emergency personnel stating that you have advanced directives so that they can look them up.
If you go to Caitlin Doughty's YT page she has a video talking about filling out this paperwork, and the example she uses is the California AD paperwork. She doesn't fill out the DNR, but she does everything else. It's pretty simple.
If you want or need a lawyer sometimes counties will have legal aid that is either free or low-cost for people who don't need major/criminal legal help, so look into that if you think you need to. Most banks and libraries will have free notary services for members, so you don't have to go searching too hard for a legal witness to sign.
If anyone asks why you, a 21 year-old, are doing this paperwork tell them that you feel at 21 you are an actual adult and need to start doing some tangible adulting, and this is just one part of a longer process for you doing the right thing as a grown-up. It's early enough in the year that you can frame it as part of your resolution for this year - to be more of a responsible adult and take care of some big adult things. If you talk to a doctor about a DNR you can tell them about your adulting journey and explain that you have taken time to consider what you want and don't believe you should be forced to undergo the process of resus because of what CPR does to the body and that keeping somebody alive artificially goes against your beliefs and wishes for your body. Educate yourself a little on the realities of resus and what goes in to keeping somebody alive artificially to prepare for the conversation so you can speak more knowledgeably. Every doctor is different so, again, your mileage may vary.
I know that there are other EMS and healthcare personnel on here, maybe they can chime in with their experiences.
Sorry for the wall of text, I just woke up and have had to take lots of caffeine to function
