Can somone please help me on this?

i went to spend the weekend with my boyfriend. found his cell phone . i never check it but had this gut feeling to do so. he was talking with another girl . she would text ive been thinking of u all day cutie. he sent i miss u , love u too tellds her i wish u can come over shed say what would we do he said i can think of a few things to do to u etc. i flipped broke his cell . was about towalk away break up he begged me to stay. i asked why didnt u delete the messages? he said he wanted me to find them .because he felt horrible. so he just lefdt the phone around me all day. i asked how he can do this to me. he said i guess i thought i needed an ego boost it made me feel good to hear. i have a self esteem problem. i forgave him. but am just so confused . he claims they are friends. but platonic frnds just dnt talk that way to eachother.i respect his honesty on telling me about the self esteem thing but im scared if he turned to her and not me in the first place . will this happen again? i am very good to him , he head no need 4 this. maybe it was just fun? id just like to hear other opinions.