BF CHEATED

Just found out the love of my life cheated on me , he said he did it because at the beginning of our relationship I had intercourse with another woman, and had to confess that was a year ago , I saw a flirty text to a number that I had seen some stuff from a few months ago so of course I had saved it . He said he done it after he found out what I did but I'm like if you have had communication with this person after the sex doesn't that mean it's more than sex ? He's told me that he spoke to her and he is done with her it's the first time this has happened on 2 years, but I don't know what to do . I really want to believe him and move on but it's so hard . We live together we've met each others families we are trying for a baby we have a dog ( 😭). So much invested ... I want to believe him and trust him again.