My ex keeps contacting me and it makes me second guess myself ?

If you feel that your ex didn't treat you well and he/she made you feel uncomfortable in the relationship then don't let him/her in your life again and hurt you again. If you feel that you feel incomplete without him/her, then go for it. But make sure to keep your distance in the beginning and try to work out all your differences in the beginning and then move forward with your relationship so that none of will will get hurt again.

I know it can be hard, but try to remember that you don't owe them (or anyone, for that matter) your time. If you don't want to be in contact with them, let them know that explicitly. Don't be unkind, but be as clear as you can, and don't leave any room for interpretation. If they keep reaching out, you can block their number and social media. If you're in school, you can let teachers or counselors know what's going on. When someone makes you question yourself and doesn't make you feel good about yourself, sometimes it's best to cut them out of your life.

Seeing messages from your ex can make certain past emotions and memories resurface, making you experience complex feelings. Perhaps it triggers old patterns of thinking and behaving. I find it really helpful in times like that to bring myself back to the present moment and let the emotions I am feeling come up if they need to, identifying how I'm feeling, what I'm thinking, and reflecting on it- the awareness of your emotions and thoughts, naming and observing them in itself provides lots of answers and eases the second guessing. This process may even help you remember why you made the decision to go your separate ways. Be patient and kind with yourself, dealing with a relationship that has ended can be difficult, and it is ok to process it as it suits you.