I've done so much wrong, how do I let him know I wont anymore?

Ok, hope you're ready for this, wanted to get opinions from complete strangers than my friends.. Ok, Ive been kinda datin my best friend for 2 years. We do our thing when we do, neither one of us do anything with anyone else, but I love him with all my heart and want to be with him for life, just neither one of us are ready for that. The problem is.. Well I have an issue with being honest with him for some reason. When we met he told me the only thing he asked of me was to always be honest and 100% with him.. I lied bout how many people i was with cause didn't want him to not like me, then it went to lying about lil things like seein an ex cause he wouldn't like it, kissed a few people I lied about. the stupidest little things like, I didn't see that message until after I talked to you when I really saw it before.. all things i knew i should have told im but didn't, then i'd lie a bout it.. We have been working on tryin to get our friendship back after I lied to him like 3 times before.. he told me it was my last chance, well i messd up again. I had to see him last night and knowin he was walking away for good and id never see or hear from him again was the worst thing ive ever felt, and we aren't even together. I always thought this day might come, but thought Id handle it better. I sat in the parkin lot 10 min crying uncontollable and couldn't even drive home. I realized at that moment, I've got to do something. Whats wrong with me? why is it so hard to just tell him even though i might lose him. i dont lie to anyone else, im not fake with other people, im very blunt with everyone but him, cause i dont want to lose him. why cant i just get things right... my main question is, how do i keep him from walkin awy this time? Ive said everything in the past, what do i do to make him see its different this time? sorry so long, and thank you for your help.. im losing my mind here and dont know what to do, i feel horrible for hurtin my best friend, i want to run away and never talk to anyone again cause i feel like a terrible person.. a liar, fake *** b****..