How do I get my boyfriend's trust back?

Hey how are you? wow what a weird weekend i had well i found out some info that has really just been soo shocking for me on Friday night as you know how i am really good friends with his mates gf well i went out to dinner with his best mates gf we were all in town and the boys were out for my BF graduation party and she really wanted to meet up with her guy unfortunately he and my ex we together and we met up and it was the 4 of us like old times.. they were quite pissed except for me cause i was driving my friend home and yeah he told her cause he was being a bit sooky that he has been trying to tune this girl from his training for the past 4 weeks or whatever and that he just got rejected… she told me this when we were walking to the car i started to cry like a baby i just couldn’t believe it only took him 3 or so weeks to go and start tuning other girls… but yeah i dunoo why he was being sooking most guys just don’t give a shit if a girl turns them down they just move on to the next one clearly… but yeah my friend said its karma and now he has realised that shit he has no one i mean i still care for him so much and seeing him on Friday i looked at him and just thought god what happened but then i guess i needed to hear that so i had closure and stopped blaming myself for it all gong wrong that there was more to it.. anyways i dunno what i should do from here now i mean i wont be calling him or anything after my friend told me i just messaged him and said congratulations i wish you all the best and take care always… i didnt want to message him and go well you moved on fast nothing for him to know that it got to me.. anyways what do you think .. lost