It depends on the diagnosis and whether your insight is impaired or not. I thought my only issue was social anxiety....I don't really like interacting with other people irl all that much turns out I had psychosis...I didn't realize I was hearing voices I thought I was getting telepathic communications.....if the way you perceive the world is broken then you can't make an accurate self-diagnosis.

If I had just trusted my self-dx I would not have gotten the meds I needed. Luckily my friends/coworkers realized something was wrong and got me help....