Life is funny - it doesn't always turn out how you expect it to. Sometimes you have this insanely amazing plan in your head that you thought would be your best life, but nothing goes according to plan. Nothing ends up like you think it will, and the funny part is that that's OK. Sometimes it's important to look past yourself and what you're feeling at the moment to think about what your future will be. How life will go on, how life will be different. How great everything will hopefully turn out. At first its hard when things begin to change, the person that you told everything to isn't there anymore, you can't text them at 2am with every random thought that comes into your brain. It's weird, and honestly a little sad. The thing that you have to remember though is that life will go on, things will get better, and if that person is supposed to be in your life they will be. No matter how much time it takes for the in between to work itself out. Your whole life can't revolve around missing and thats something you need to realize.
The first step is to just understand why you or the person you miss decided to stop being around. Realize that its probably for the best and be happy for the person you miss. Don't wish anything bad for the person you miss, thats not fair. Sometimes life just makes people drift apart, or sometimes you both need to work on yourself and thats not something to be sad about. You are both just trying to give yourselves your best chance at a happy life. Keep yourself busy, and surround yourself with other friends, give yourself the opportunity to branch out and meet other people.
The next step is to remember that your whole life should not revolve around another person, this is your time to be a little bit selfish, do something you've never done before but have always wanted to. Everyone makes goals in their heads of what they want to do, but a lot of the time people become too scared to do something or feel like it is unattainable. This is your time to take risks, to achieve your goal, to buckle down on what you have to do to achieve it.
The final step is to remember that it is okay to miss someone sometimes, it is okay to wonder about how they are doing, wonder if they are safe, wonder if they are happy. Sometimes there are people that leave a big impact on you, and thats an okay thing to miss them. They will creep in your head, and if you ever miss them enough than maybe seeing them one more time is just what you need. Sometimes closure is the most important step of moving on, or its the most important step of realizing the person that left your life should have stayed, and you can fix things.