Vertimas - Prancūzų-Anglų - Si j'avais su qu'un jour je te perdrai

the correct translation is :
If I had known that one day I would lose you, in my turn, I would have given everything to keep you.

Yes, you are correct.
Unless there is a context, though, the extra phrase "a mon tour" ("in my turn" makes no sense, at least to me.
stevo