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Even I'm happy to have her as my girlfriend.
Literal
I [too-だって] her [object-を] lover [as-に] can-do-and happy [explanatory-んだ].
彼女を恋人にできる uses the pattern [object]を[state]にする/できる — 'make X into Y / be able to make X into Y.' 恋人 ('lover, sweetheart') is a more committed term than simple 彼女. The て-form connecting 'できて' to 'うれしい' chains cause and effect: 'able to [X] and [so] happy.' A classic cause-based joy statement.