The lesson here is
not what you have lost, rather, the significance why you have lost
it.
If you were to reverse time and materialize as a third person,
trying your best to talk to your then-mindset as another party,
nothing will sink in. Going back time is merely a 'waste of time'
because 'the-back-then-you' is bound to lose this girl in exchange
for personal growth and cosmic lesson.
Regardless, it is impossible to turn back time.
What you are doing now is attempting to look over your shoulder,
hoping you can catch a glimpse of her presence behind you. However,
it's futile for, she has already moved on so far. In her mind, to
return back to where she came from previously makes no sense to
her. It makes sense to you because you are regretting... but would
it be so for her?
Considering your behaviour and action, shouldn't she be glad to
move on? 
P.S: Learn to
live with your regrets and channel them into force to mould your
future, not something to repair your irrevocable
past. 
Cheers
