I beg u all please help?

why is it that ever since i found out i was 5 months pregnant & ever since i had my baby my bf is not the same guy he used to be i've caught him talking online to random girls & getting their number also he has lied to me a lot & has given me the excuse that he don't tell me certain things cuz he knows i will flip out & that he needs his privacy cuz he starting to feel cornered i have caught him watching & subscribed to sites & some sites he has payed 1.00 for a 2 day membership & even though he no longr is chatting with oline rls he met a girl at work she doesnt work there but shops at the store a lot thats what he said but she taken but he talks to her he erased her # cuz i confronted him about it but i snooped through his ipod touch & he put it in his notes thinking i wouldnt find out he hasn't been honest about how he actually met her...i've confronted him so much that i feel its making it worst so i'm gong to write a letter to him but i have no idea how to start it & stuff so please i beg everyone help me write this letter that will make him realize what his doing i dont want to sound like im trying to control him or im keeping an eye on him i want him to change his bad habits & want him to know im hurting