I'm trying to make sense of this phrase to me
Kimi o shiawase ni dekiru nowa boku dake da to shinjite imashita.
which I found translated as
I always believed I was the only one who could make you happy.
"kimi o shiawase ni dekiru" is "be able to make you happy" or something like that
shinjite imashita is "believed"
"boku dake da" , "I just am" (I suppose)
Question is, is the "who" given by some word or particles (such as nowa) or is it just the structure of the sentence who puts a "who" when translated there , and which meanings/functions have the "nowa" and "to" particles here?