how do i deal with this

i had been with my partner for 8 years from the young age of 17..we have a four year old son together and last year he left me for the receptionist at his work and moved in with her...my whole world came tumbling down around me and i couldnt cope so i started going out with friends and doing the things i had missed out on for so long..3 months after he left they broke up on xmas eve and he of course came running back to me..numerous fights later i couldnt fully commit myself to him worrying about him going back to her so we didnt see each other for another 3 months and during that break i was seeing someone and fell pregnan..my sons dad has no idea i was ever with anyone else and i fear telling him im pregnant..i do love him more then anything and im now 7 months pregnant and he hasnt picked up on it..i need to end things permenantly with him before bub comes along..but the fear of never having him again kills me..i know i am wrong but i dont know what to do or how to cope with this