I want to ask, my girlfriend's reply to text messages has become very slow. Holding a mobile phone all the time, I always feel that she has someone else in her heart. What should I do?

At this time, the best way is to communicate with her, tell her what you think and tell her that you love her. If she still loves you, she will definitely explain it to you, so whatever it is, give her time and tolerance. If she doesn't love you, then just give her a chance and give herself a chance to let go. After all, the phenomenon you mentioned is not normal.