、。
When I managed to become friends with her, I was really, really happy.
Literal
Her [with-と] friend [into-に] can-become-past time [topic-は], that was-very happy [assertive-よ].
A straightforward memory, with one small intensifier that's easy to miss: the それは before うれしかった is not a demonstrative ('that') but an intensifying adverbial use — 'that is to say, very much,' 'I mean really.' It's comparable to English 'it was, like, really happy,' or 'let me tell you, I was happy.' The potential form なれた ('could become,' 'was able to become') carries its own emotional weight: the speaker didn't just become friends — they succeeded in becoming friends, implying the achievement required some effort or was uncertain.