There's a huge difference between wanting someone because you genuinely miss them, and only wanting them because you're lonely.
Missing someone is longing to be with them. It's your heart aching, your stomach knotting at the thought of them constantly running through your head. You can't eat, drink, or sleep without a picture of them in your mind, and they're the first and last thing you think about each and every day. You went from constant conversations and frequent contact to nothing at all in what seems like a fraction of a second, and you were left empty.
They probably said things like: "we shouldn't talk for awhile", or "we need some space", and "I just need time to get over you." If your heart was made of glass, those were the words that cut right through. As time goes by, it seems like you're never going to find something that was even as close to what you had with them. All you want to do is talk to them and tell them how much you miss them; all you want is for things to go back to normal, but you know that they never could.
Your loneliness winds you up in the arms of those who disguise lust as love, and the attention they give you is toxic, but you'd rather be poisoned than feel alone. While you're intertwined with an unfamiliar face, you know you'd rather be with the one that left you. Eventually, you can't take the lonely nights, and you give them a call, not because you miss them the same way you missed them before, but because you can't stand being alone.
That's the difference between wanting someone back because you can't stand to be without them and wanting them because you can't stand to be on your own. Just know that before you press the call button- if you actually want to fight for them to come back or if you just want to play a game of back and forth. Don't confuse your lonesomeness as your willingness to make things right. So before you say "I miss you" make sure it's the real you, not the person you turn into when you're alone.
