You need a Durable Power of Attorney for Healthcare, in addition to a Living Will. (My personal suggestion is that your SO not be this person, nor any of your relatives.)
You can get these blank documents, samples, suggestions, and directions from the social work department (or a similar name) at any large hospital, or check your library for a Nolo law book which will have the forms in it, or from any large stationery store. Or visit www.nolo.com
for everything you want to know.
Complete them, make appropriate copies, have copies also placed in your medical records at every place you receive healthcare.
Additionally, write and sign a letter to the person serving as your DPOAHC, restating EXACTLY what your wishes, are, what limits you wish too place on medical interventions, etc. Make sure this person has copies of all three of these documents.
Make sure your relatives are aware of your wishes, the identity of your DPOAHC, and where these documents are located.
So far as a Last Will and Testament is concerned, you will need an attorney for this. You could do one yourself, again through Nolo, but if any of your relatives chose to challenge it, there could be problems (though you would be in no position to participate in the challenge).