A friend is, as it were, a second self.

Close friendship works like an extension of your own perspective. Somebody who knows you well can hold the parts of you that you have temporarily lost, remind you what you are like when you are well, and see your situation from where you stand without being trapped inside it. That is why losing such a person feels like losing a piece of yourself rather than losing company.

If you have somebody like that, the maintenance is unglamorous and mostly consists of being in touch when nothing is wrong. If you do not, it gets built by time and repetition rather than by locating the right person. Start with whoever you already like and see often.