Conditionals to express an action in the past if a condition been met

Can anyone please explain me if the following two sentences are grammatically correct? If so, how they differ in meaning?

If I had gone back when I was unemployed for one year after my graduation, I would have invested my time in many productive works.

And

If I could go back when I was unemployed for one year after my graduation, I would have invested my time in many productive works.

In the first sentence, I have used past perfect ("had gone") in the subordinate clause, whereas in the second sentence, I have used past indefinite ("could go") in the subordinate clause.

I am trying to express a feeling that I want to go back to a certain time in the past so that I could invest my time in that duration.

If I had gone back... suggests that you wish that, at that time, you had returned to some unspecified place.

I think what you are probably trying to say is If I could go back to the year when I was unemployed... I would invest my time...

English conditional tenses are not particularly suited to talking about time travel. The way I would express this sentence is:

If I could go back to when I was unemployed for one year after my graduation, I would invest my time in many productive works.

The problem is that if you can go back in time, the hypothetical period when you've gone back in time is in the world's past, but in your future, so you have to decide which perspective you are looking at the situation from before you can choose the tense. I think the natural choice is from your own perspective.