Moving on is a difficult thing to do.
While the phrases, "you're better off," or "it was just time," and "on to bigger and better things," can all be true for a myriad of situations, that does not change the fact that it is difficult to move on. When something or someone is important in your life, you don't usually want to let them go. The issue here is just because someone was important to you once, during a stage of your life, that does not mean they are meant to be there forever.
People and things come and go. This is a fact of life and the sooner you accept it the sooner you will be able to deal with it. When we are young, we seem to have this idea that our best friends will be there forever, no matter the distance or changes that may come. The lesson we all have to learn as we grow older is that people change and you won't always be at the same stage of life with people you once were. Things you once found fun to participate in or issues that once were important to you might have a passion that fades.
The thing is, this is normal.
The other thing is, how you learn to handle people and passions leaving your life will define you as a person.
It can be hard not to be mad at a friend when they start pulling away, getting too busy to return your calls, or move away and don't keep in touch. It can be hard to realize that you just aren't willing to put the time into a passion or project that you once were, and maybe it is time to move on. This is why we all must develop some perspective. See, I am not telling you to run at the first sign of trouble in a friendship or to head out the second the going gets rough on your passion project. No, what I am saying is that sometimes you get that gut feeling that it is time to move on.
Listen to it, no matter how hard it may be. Then think about your next step and try your best to move on without resentment. This may sound like I am preaching to the choir, but it is so easy to harbor little resentments in our lives. We hate the boy who didn't give our relationship a chance, the girl who was too busy for us, the sport that we loved but could no longer pursue, or the major we ended up changing. Those little resentments take up a part of our lives; they are a part that we can reclaim. Moving on can turn from a negative to a positive. That relationship that didn't work out? Now you can find the person who you are meant to be with. The friend who was too busy? That gives you more time to focus on the friends who make time for you like you make time for them. The sport you had to leave? Coach, watch it on TV, join a rec league or find a new athletic adventure. The major you had to change? Well, it probably wasn't your passion so get excited about your new one.
It is easier said than done to "just get over it," but moving on is something that we need to encourage ourselves to do. Don't hold on to these little resentments, cherish the time that you had and then just keep moving.