How do I know if someone is "the one?"?

How do I know if someone is "the one" for me? There are some things this person does that makes me disappointed and just, sad, but other times we are so happy. I feel like I could potentially be settling because they accept me for who I am, with my flaws and mental instabilities, and such. I feel like this is the person I want to be with because I will not find someone else who accepts me for me. I know there are people that I feel like are more attractive, and people that I feel like I connect with more. But I don't know if I'm going to find someone who accepts me that is better than this person. How do I know if this is the person I want to spend my life with?