found out my bf was cheating

i (18F) just found out my bf of a month (18M) was talking to other girls while we we're together. the girl he talked to was 9 days after asking me to his girlfriend and up until a week before thanksgiving. i can't remember the way the messages were bc when i saw it i panicked but they were flirting in a way. i'm very hurt and very confused because even after i told him my relationship trauma he still did this but also he's been with me almost everyday since we started talking back in October and he hasn't talked to other girls since that week before thanksgiving. i didn't break up with him, but i genuinely don't know how i can trust him after this. any advice on this ?