Unfortunately it seems he has moved on and so should you. I realize you may want him back but there is not much you can do nor should do as he has decided at this time he wants to be with this other person and you are just going to have to accept that, let them be, let go and move on.
You should not. Just face it. you need to move on.
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.
get a new girlfriend or boyfriend
Depends on if your boyfriend wants HER back. If he does, let him have her. If he doesn't, don't worry about her. comfront her and tell her to back off. if she don't kick her ass.
ask your boyfriend to tell her to back off while he's going out with you and confront her your self with your boyfriend and talk about it.
You should make her jealous and have a battle between you and her current boyfriend. Make her realise that your worth another try.
No, he/she is not a good boyfriend/girlfriend. Leave him/her and wait for him/her to come crawling back!
Find out if they have a girlfriend/boyfriend. If they don't, you are free to ask but if they do have a boyfriend/girlfriend, you should just be friends for a little bit. When they break up with who they were going out with, give them time to heal then go for the move.
get over it
You should not try to get her back. You should try to get over her. If she is happy then you should also try to move on.As heartbreaking as it is for you she has another boyfriend and you cannot make someone do something you want such as your girlfriend coming back to you. Be wise; get out with friends and start dating other girls and if your girlfriend sees you dating another girl it may just make her want you back (that's a maybe) or, you will find the girl that you really should be with.
Unfortunately he has moved on and so should you.
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!