There are too many memories
Anytime you talk to your ex, you will think about something, be it happy or sad. Some memory will come to your mind and you might want to experience it again but knowing that it is not possible will hurt you.
Your ex might be moving on and it can get frustrating for you to see
Seeing the person you shared a special bond with dating a new person might make you feel jealous and sad as you are not together. Also the fact that they have found someone else can make you feel uncomfortable.
It might make you anxious or depressed
Talking to your ex might evoke feelings that you are trying to let go of. And not being able to talk it out or convey it to them might make you feel anxious or depressed.
Remember why you broke up
Before you contemplate getting back with your ex, remember the reason you broke up in the first place. The fact that you are not together anymore and the bond you shared is in the past, you would not want to relive the memories all over again.
You did not resolve your issues
If the reason why you broke is not resolved or talked about, there is a chance that they would get repeated and there would be changes in the relationship because of it.
Missing someone does not mean you still love them
People often confuse missing their ex with still being in love with them. The break up is fresh, the wound is new, and it hurts, but it does not mean that you are still in love or even want to get back together.
The trust you once had has been shattered
Trust once broken is very difficult to rebuild or put back together. No relationship works without trust. Going back to the cycle of mistrust will do no good to either of you.
The wound is fresh and you are vulnerable
The break up is fairly recent, and the wound is still fresh. You are extremely vulnerable right now, and getting in touch with your ex again will only make it harder. Remember, it takes time to heal. Give yourself that time.