When it aches, what does it mean?

How do you let go of something that is the past and long gone? It is so much easier said than done and definitely easier to tell others or give others advice but when it comes to yourself, it seems as if it is the impossible. There will be times where I think that I'm all good and that I've let go of the past but there are still times when I see or hear things that remind of me the past, I'm still aching. Is that fair? For me and for the people around me? But why am I hurting of something that is long gone into the past and something that I was never a part of. Is it normal to be like this or am I the only one whose like this? It seems like its uncontrollable and I'm hurting. Could this be something that's my problem?