Yesterday were opportunities, today there are other opportunities. Focus on today. Life is NOW.
Yes, there was Horace that said "carpe diem", "seize the day". We regret about the past and don't see what today can offer to us.
Yeah keep on regretting then you will have nothing. Just focus on what you have today and hope that you will get more something out of it. Don't put yourself in a situation that you will never win.