I am losing my mind, am i overreacting here?

My partner emailed his baby mama behind my back. On the day i found out he had cheated on me for yrs with this woman, he promised he wldnt have anything to do with her & the child. I'd always lived with him (unmarried) with a child. She had ended with him when she found out about me.Since he cut contact with them she had been haring us both by emails/texts, he promised me if he ever felt the need to communicate with her he would show me their communication.Yesterday i caught an email he sent to her saying "I cld have got injunctions against you for haring me & other people but i didn't because i know family is for life" I am really mad, wouldn't you