Cheat??

So my boyfriend accidentally sent me a message with three hearts on the end, I was asking him who it was for and he didn't tell me, later on he told me after me begging all day. Turns out it was for his 'mate' who's a girl. Thing is is that he has a history with this girl, sending pics, talking, meeting up etc. The day before I met him and he said he would delete her but when j was gone he added her back. I have full on fallen out with him about this. Is it me over reacting? I can't help but worry that he prefers her over me. I know she likes him as well which makes thing 20 times worse. Also she doesn't like me... Please give me your thoughts on this I'm struggling aha