She was indifferent when I took the initiative, and she contacted me after I disappeared. What is the psychology of such a girl?

For this problem, you are probably the legendary spare tire. The girl's attitude towards the spare tire is that if you are too active and enthusiastic about her, because she has her real boyfriend after all and can't give you a fixed job for the time being, she will be cold and distant from you. If you suddenly disappear and ignore her, she thinks it's a pity that the spare tire is suddenly gone, so she will suddenly wave to you, hoping that you can come back and stay in your spare tire position honestly, so it's generally such a mentality.

Some people may not accept this behavior of girls, or say that they are in a spare tire position. Then it is no problem for you to leave resolutely, but before getting married, everyone has the right to choose. Even if she has a real boyfriend, it doesn't mean that they will be smooth sailing and prosperous, because after all, they are still young and everything is full of variables. She may spend some time with her boyfriend and find that they are not the most suitable, or she may be with the spare tire in turn. So, if you really like this girl and think you two are very suitable, you might as well continue to be friends first.