My ex already has a girlfriend, but he said he would come to see me when he has time, but I occasionally send him text messages, and he doesn't respond very coldly. What's his mentality?

When he has time to see you, he wants to make it clear to you that he has a girlfriend now. In order not to make her sad, jealous, or even conflict with you, he doesn't contact you anymore, so it's very cold not to reply to your text messages, because he wants to make it clear when he meets you. Don't be sad, it will be better to start a new relationship.