Engaged, sad and insecure

I'm engaged to the love of my life. I love him so damn much. I felt lucky that I finally found someone who made me feel happy until one day... He was driving and asked me to text someone. While I was doing so, I saw some messages from his ex insulting him and me. Apparently she wanted to go back or something. I got really mad about it and became suspicious. After that day, I was feeling really insecure. I decided to check his phone in secret and found out that he had been flirting with her and stuff. She even sent him a sexy picture once.

I really dont know what to do. I wish I could stop feeling sad, angry and insecure. He's been apologizing about it and he "swears" he didnt have sex with her nor kissed her but... I dont know. I want to believe him but at the same time I feel so stupid because they met in secret one day he took her to the hospital. So maybe that day they did something.

That happened some months ago and he's been trying his best so the relationship works out. We are going to get married in two months and our pre-wedding photoshoot is next week. I still feel soo bad about it but I love him so much. I dont wanna leave him.

I cant control my emotions. One day I'm happy, but the next day I get angry. I simply cant get over it. How can I make this relationship work? How can I forget what happened? How can those events stop affect me?