Question: How will I deal when s/he is with another person?
Answer: Honestly, you probably won't deal well. Even if you initiated the break up seeing your ex with his or her next can be a shocker. Afterall, this person used to be yours and you used to be the one at his or her side, now there is a new pair of eyes gazing lovingly into his or hers. The important thing to remember (and this is hard) is that you two are no longer together and who your ex is with is not at all about you. This really has nothing to do with you or your life. Sure it may hurt, sure you may want to scream or cry, yes you will want to run away and hide, but in the end this is not a slap in the face directed at you, this is your ex having a life.
One warning, if the other person had anything to do with your break up (as in cheating or acting as confidant to your ex while the relationship was in the dumps) expect to feel angry. You have a right to feel angry. Expect to feel betrayed. You have a right to feel betrayed. Expect to feel hurt. You have a right to feel hurt. But do not under any circumstances freak out and cause a scene. This will do no good to anybody and you will regret it. If your ex was so callous as to involve another person in your break up do you really think that s/he is going to care more now than s/he did while you were together? This person was a factor while you were in a relationship, while your ex was supposed to care about hurting your feelings, why would that ex suddenly care more now? It isn't logical. S/he was with this person and with you at the same time (emmotionally or physically, it doesn't really matter) this new person is not really new at all and you probably already had an inkling that they were together. Take a deep breath, leave if you need to, and cry or freak out in private far away from your ex and the next. Let out your hostile feelings but don't let your ex see them, no good can come from it. If your ex thinks s/he made a mistake with this other person they do not need to have you freak out in order to realize it, they will come around on their own and then your ex will have to deal with his or her own bad feelings. A scene starring you won't change a thing, keep calm long enough to get somewhere safe and private and then vent it out.
As a general rule seeing a person you were once intimate with in an intimate situation with another person hurts. It hurts like heck. It is unpleasant in ways you can't understand unless you have already dealt with the experience. Know it will shock, hurt and confuse you. Know that any negative feelings you had during the break up will come flooding back, even if only for a few minutes. Be prepared to not be happy. But know that this is easier than dealing with a break up and you have already done that. The hardest part is over. Take this type of a run-in as a small step backward and don't deny your hurt feelings. As with all the bad feelings about the break up itself, this too shall pass. And take comfort, there is only ever one surprise meeting with an ex and their next. Phew!More Teen Advice Q&A