Remember the first time you had almost drowned to death? I do. I don't remember who did it but they were trying to hold onto me and use me to float because they were afraid of the deep water. I could have died that day. Later i told my father what happened and he told me one of the most useful pieces of advice. Instead of trying to fight with the person and use up energy, instead let them hold on to you and sink to the bottom. Once they let go, you can rise up to the surface saving yourself. And then be able to save them. 

Some people will drag you below the surface whenever possible. They will continue to do this until they have used all your resources and then move on to the next person that has what they want. Not all people do this, but I have encountered more in my life than I have wanted. You just have to let go. It may be hard because they seem like your closest friend or post that you rest your head on. But it's imperative you take care of yourself first before trying to assume responsibility of someone else. Otherwise not only will you drown, but you can drown the other person as well.