How to understand this sentence? Please help me!

I lost my identity in that of the individual whose life I was reading.

how to understand it, especially"in that of "

Please.

I lost my identity in that (identity) of the individual…etc.
Meaning – this person lost his identity in that another person's identity.

in – preposition
that – is a word that replaces identity (relative clause)
of – is a preposition that shows belonging of identity to the individual from the second part of the sentence 

in that of = in identity of

I hope you'll understand what I mean :)

 I was reading the life of an individual .. I lost my identity in the identity of that individual..

 it means that you just incarnated his life , put yourself in theirs .. and when you wanted to go back to your real personality , you got lost..

 Hope it will be useful!!

Thanks so much for the answers by you two.

if it were not you, I would not make sense of it.

Thank you so much.