The point isn't to lower your expectations, the point is acknowledge that the way you feel is under your control and it's up to you to do something about it. So if your coworkers let you down, they don't make you feel let down, your own expectations make you feel let down. How you fix that is your responsibility.
You can try and help your colleagues, you can rely on them less, or you can quit your job. Blaming your colleagues and making them responsible for your feelings means feeling shitty and feeling powerless to feel better.
> John didn't let you down, you had expectations about what he was going to do
This is where we disagree. John did let you down. It's OK to blame him. It's OK to be a normal person that reacts to other people's actions and not some kind of zen robot. And recognizing that someone else's behavior causes you to feel shitty doesn't make you powerless to change.
John did let you down, but he didn't make you feel let down. You can blame John for letting you down, but you can't blame John for you feeling let down.
You are responsible for your own emotions. Recognizing that doesn't make you a zen robot. You can still feel. But don't put those feelings on other people and make them responsible for making you feel the way you want to feel.
You can recognize patterns for your emotions, like "I get sad when people make fun of me", but that doesn't mean that you have to be sad when people make fun of you. Recognizing the distinction is what gives power to change.
And sure, there are limits. The feelings that arise when tortured for example are rooted in physiology and near unavoidable. But I suspect that John didn't let you down by waterboarding you.
Even Vulcans can't quite manage this level of emotional control. Complete personal responsibility for one's own emotions is a nice ideal, but humans are not stoic robots. People are responsible for their behavior and its effects.
You can try and help your colleagues, you can rely on them less, or you can quit your job. Blaming your colleagues and making them responsible for your feelings means feeling shitty and feeling powerless to feel better.