Im so conflicted and torn not sure what to do?

Brief overview....live wif bf year and half, he texts a younger woman inanother state he has kept all 3000 texts that they share, some very flirty, she does not know that he has live in gf. Do not judge me for snooping but yanno when you get a feeling in the pit of your stomach like i know he is lying well i got that feeling about three weeks ago and yep I looked for evidence, NOT right but I looked. Anyway we had a discussion on her and I told him he needed to tell her he was in a committeed relationship and not avoid the question when she asks if he has any hot chicks or dates for the weekend. Im not asking for him to cut ties with her, im asking for him to let her know the score. That he loves someone. He agreed after some discussion. three weeks went by and one day i asked if she had texted him, he said no she has not and its been the longest in over a year. Well we all know that they is a delete on on cell phones for text messages, but i took him for his word. I really dont know what got into me yesterday but i text him and said i found her on fb and messaged her that i was your live in gf and then proceded to ask him something. He called addressed my question then just when i felt he was gonna address my comment his cell phone went off and he had to go. One thing I have to mention here is right now I am going thru some medical problems and seeing specialists and docs ect. So i had appt with dr yesterday afternoon i texted him i was off to dr and wish me luck he replied with kisses. After appt i text him i was done and i expected him to call me and ask me about appt. No phone call. he called me on his way home asked but seemed distracted. He made no comment about me telling him i found her on fb ect no lecture no reprimand nothing. Something just didn't seem right yanno and yep checked his phone sure enough they were texting all day to each other, kindof ironic after three weeks of no texts they contact on the day i tease him about fb. She asked what he had been doing (great opening to mention that he and my gf did so and so) nope no mention just said he has been busy its the season. I really am not trying to make a mountain out of a mole hill here, but this is important to me. Some of you are gonna say kick his *** to the curb, its an option i must admit but I believe that you try your hardest to make relationships work and im not getting any younger lol. I love him and he makes me laugh and we have a love of racing together. Im just not sure how many chances I can give him before i say enough is enough. What i want is ideas on how 1. i can make him see what he is doing. and 2. how can i get him to be honest with me about this relationship he has with her. and 3 where and what do i do know? am i staying because this is a challenge to me?? i really am torn as to what and how i handle this right now, any advice would be great. thanks for listening