The Butterfly Effect is a theory that states if someone goes back in time and tries to change the past the outcome will be different. Of course time travel is impossible but that theory holds true to us.
Your boyfriend of ten years just broke up with you. You wish that you could get back together or did something to make it work. It if did work the outcome of the future would be different. You might miss out on someone even better than your ex.
Say you wanted to get a dream job across the country. The one you love wants to stay here. It turned out you weren't qualified for the job. If you took that job you and the guy you are with would have to break up.
If you missed a chance with someone or someone let you get away don't get upset. Sometimes people come back. Ever hear that saying "If you love something let it free, it if comes back it's yours and if it doesen't it wasn't meant to be"? If you are meant to be with an old love it will happen. It might not happen when you want it to but it will happen on the universe's time table.
You never know what's going to happen or who you are going to date. Just have faith and if something is meant to be it will happen. Don't try to control it or get stressed out because if a relationship does not work out it only means that there is someone better.