Never regret not being able to realize certain things in your life. You are who you are now, partly because of these things, and if you are satisfied with your existence now, there is no need to feel sad about past. Maybe even a small difference in the past could make you feel displeased now. The same is true with regard to the future. You are going to be someone, partly because you cannot achieve all of your goals now, and nobody can know if these achievement can make us happy or cause suffering in the future.