how to do this...

i have been with my baby's dad for 4 years now. last month we broke up. I had sex with my neighbor using protection but the condom broke. that happened 23 of last month. today u was diagnosed with Chlamydia. my baby's dad and I had sex a few times after 23 of June. he asked me a few times if I had sex with someone else and each time I said no. but now... I don't know if telling him the truth and lost him forever. or lying and leaving it for the luck to decide. I know he will leave me if I even had sex with the neighbor.