Your second version just sounds more natural.
None of them are incorrect, per se, and there is nothing wrong with your second version
Our whole life is a struggle to return to ourselves.
Your first version uses "return" as an abstract noun.
Our whole life is a struggle of return to ourselves.
This is also grammatical—we say "A person of interest". However, "return" is harder to read when it is a noun and not a verb.
For instance, we still use "meeting" and not "meet" because "meet" is not distinct as a noun. Therefore, contrasted with the alternative, this version sounds clunky and almost wrong because it is not phrased as follows.
Our whole life is a struggle of returning to ourselves.
Thus, I think the second version sounds simply natural. Though, the first version is a close contender after the rephrasing. If we are being more lenient, they are both grammatical.