I need help

Hey ladies I need you guys help. I'm going through something and I don't know how to respond or handle it. So I have been with my boyfriend or fiance ( I'm no sure which anymore) for almost two years now but I have known him for almost ten years now. We started off really good and for the last 11 months ( i thinking ) we have been fighting over the littlest things ever but recently he started talking to his ex fiance again and he won stop no matter what I say or do. He said I was all innocent and stuff but I went through his phone one day ( I know not okay but I had his phone cause he said I could go through it and no matter what he goes through mine) and I found it wasn't okay. They were flirting and saying I love you and had pet names for each other and she sent him inappropriate pictures. Then I told him he had to stop talking to her and he did for a day. Then he kept talking to her and hidding it from me. Then he went behind my back and made plans to see her like 4 times and he says I have no right to be mad cause they are just plans and it always falls through. I told him I don't want him to go see her and he said if he ever has the chance and the money he wants to go see her. I told him if he did that I was gone. He said it wasn't worth losing me over an ex but he still talks to her. I don't know what to do. Help me