How do I stop reacting on emotions

like I’m not with this girl but I get jealous that she’s talking to someone else Yh I know it’s stupid . And if I reacted it would blow my chances in the future . I have remained cool yet . I just don’t want to react on emotions again because she got suspicious when I asked her why she takes ages to reply . So she said “ it shouldn’t bother you if I’m not with you” so I just brushed it of with it’s a bad habit that Iv had with my mates