SO's friends

So, i just need to vent, and get some guidance i guess. My man, has always given me problems with his lady friends. He claims they "arent even that important" to him yet i always find him messaging them, sometimes first, or such. It just doesnt sit well with me, especially the way they talk to him and how he responds. Im really trying to work on my self esteem (idk where to begin quite frankly) but also within the last couple days hes broken my trust. I just dont know what to think, how to even begin to trust him, or anything. it's not that i don't want to, i don't know where to start. hes been trying to prove that he means what he says ("theyre nothing" especially after the last few days)but a part of me is nagging at me tellong me he'll do it all again eventually. he's really a sweer man, just stupid sometimes. POSTTIVE/ PROGRESSING advice please.