Thats impossible
Have you tried flirting?
If the boyfriend or girlfriend hold you back from your purpose.
No, you should not email your ex girlfriend to try and get her back because she has another boyfriend. You had your chance to make the relationship work out with her and since you didn't then it time for both of you to go your separate ways.
You don't ! Your relationship with him failed for whatever reason. He has a new partner - you have no right to interfere in their relationship in the hope of winning him back !
No, he/she is not a good boyfriend/girlfriend. Leave him/her and wait for him/her to come crawling back!
A girlfriend can trust her boyfriend by asking him if she can or be testing to see if she can. A girlfriend can set up some simple experiments by saying things that she already knows is true and then testing to see if her boyfriend is honest with her. Also, trust takes time and if the girlfriend has lost her trust in her boyfriend then he's got to get it back. Or he will be costing their relationship, big time.
get a new girlfriend or boyfriend
If your girlfriend has broken up with her boyfriend then take the risk and either phone her and go see her and ask if she'd like to go on a date and then take things slow and easy and see if your relationship picks up again.
get over it
if your girlfriend cheated,you should end the relationship
I would advise you to talk to your ex about it first and see how she feels. If she doesn't feel the same way, then go back to your girlfriend or boyfriend, or depending on how strong your feelings, it would be best to break up with your girlfriend or boyfriend, anyways.
If he has a girlfriend, don't go back. He will do the exact same thing to you, that he will have to do to his current girlfriend. Good Luck!