I'm going to tell my girlfriend I saw her ex with a girl and that they seemed happy together.
Are there specific reasons why you want your girlfriend to know about this?
No, I just want her to feel jealous.
People should be prepared to move on after relationships, and they shouldn't be concerned about whether their exes are dating. It's wrong for you to try to make your girlfriend jealous of her ex, when you should be concentrating on making her happy to be with you.
I'm going to keep doing it until she is the most jealous she can get. It's going to work.
It's wrong to try to make her jealous and want her to feel that way. You need to support her and be there for her not mess with her emotionally.