"defining" doesn't lead to bias. you are left-leaning regardless of what you decide to call yourself. The important thing is recognizing and acknowledging your predisposition, which it seems you have.
what it protects against is group ideology. I don’t want to start thinking things simply because the political party I align most with thinks that, and unfortunately, identifying with a political group can make people start doing that. I do recognize my biases when making decisions, it's important to do so, I just find it becomes harder to do that when I start identifying with a group
People are prone to bias regardless of their political identification. Identifying as left-leaning provides no more protection against bias than any other political identification.
what it protects against is group ideology. I don't want to start thinking things simply because the political party I align most with thinks that, and unfortunately, identifying with a political group can make people start doing that.
The left isn't anymore a political group than leaning-left is. Political parties have positions on the political spectrum, but positions on the political spectrum are not inherently political parties. What left political party are you referring to?