My girlfriend's ex-boyfriend always calls her and sends messages, which makes me feel bad.

You can be sure that you are sincere to her, but you still don't understand, is she sincere to you? If she is sincere to you, I don't think she often contacts her ex-boyfriend, especially her ex-boyfriend sends her ambiguous messages, which is enough to show that she doesn't really love you! Of course, everyone has the right to choose love before marriage. I mean, if your girlfriend symbolizes your true love, you should redouble your efforts, especially in life, you should be more considerate, communicate more and deepen your feelings slowly. For yourself, you should give her a sense of security, that is, work (study), dedication, responsibility and so on. Only in this way can you get her true love, don't you think?