Can diagnosis make someone more ill?

I wasn't that bipolar most of the time until my bipolar diagnosis.
Things got worse.

This could be because I allowed myself to identify with bipolar. This
could be because I allowed my 'bipolarity'.

This could be because of the power of words. This could be because I was
told I was something I thought I was that thing so I became that thing,
i.e. my bipolarity got worse with a diagnosis of bipolar affective disorder.

This could be because of other things I don't yet understand. The
influence of areas of science rarely studied, such as the research about
blue tits and milk bottle tops (a study I haven't read - it may just
have been a TV program) that showed they learned how to get into milk
bottle tops really, really quickly - far faster than science predicted.
What I mean is that there may be unstudied effects of diagnosis that
make account for the mechanisms involved in how a person becomes more
disordered based upon diagnosis or self-diagnosis.

A friend of mine mentioned to me that when she had to read DSM as part
of her psychology course she ended up giving herself lots of diagnoses.
I think she understood that there's a difference between how the
information looks on paper and what it's like in real life, i.e. that in
clinical practice the important thing is about severity. I think that's
right anyway.