1. You should never, ever date two people at once unless they both know. You haven’t given me information that would lead me to believe they know OR they don’t know, but if they don’t, this is bad. It makes you very untrustworthy. That aside, you really should break up with the second. Dating someone else is bad enough, but dating him without having feelings for him for months is also leading him on. You shouldn’t do that to a person. Just let him down easy, and make sure you do it in person.
2. It’s completely normal. Breaking up doesn’t automatically diminish your feelings for him. You just have to learn how to move on from him with time.
3. First, break up with your current girl. If you go after your ex while with someone else, she’ll feel like a second choice and won’t know if she should trust you. Also, be understanding. If she says she doesn’t know if she can trust you again, tell her you understand that, and you’re willing to work with her on her terms to gain that trust back. Be there for her, and show her you can and will be good to her this time around.