You’re starting to become a loner - welcome to the adult club. Priorities change, people adapt and you fall off their list and they fall off yours. No biggie. None of this is a reflection of you as a person. Perhaps you might have outgrown them, or they outgrown you. Either way, do not misjudge yourself. Your social circle should be people that inspire you. Doers, thinkers, believers. It’s better to have a small, smart social circle, rather than a big crowd of fools. Don’t worry about people walking in and out of your life.
Many times we tend to misread people’s minds about us and presume that they like us when they don’t. So if you are doing this, please stop. Giving someone too much attention is detrimental not only in the energy that it drains you of, but also the fact that people push away when they get the attention. I suggest you step back from relationships and let things flow. Work on yourself and be content with yourself, you will attract the right people and situations. Loneliness is universal and gets us worried for ourselves, but turn your loneliness into a means to re-assess and discover yourself. =]