too many chances?

So I've been with my boyfriend for a couple of months over a year now and I just want to ask, how many chances are too many chances?
I'm 20 and he's 25. He's my first relationship, my first love, basically my first everything..
Towards the third month I learned he had been lying about doing drugs, I stopped because he told me he stopped, he lied to my face while he reeked of it, I let it go once. Then twice. Three times. Four times, every time he's "sorry". 
I tried bargaining with him saying it was fine as long as he would tell the truth. Big mistake. 
We had been having a rough couple of months due to other things he's lied about along the way..
When we saw each other and we're trying to work things out I asked him and he lied about it again to my face. 
I knew he was lying so I pressed on and asked 4 more times. FOUR more times. Then he told me the truth. 
I asked why the hell is he lying about it after I tried bargaining AND was trying to fix things. He then said he didn't lie to me and told me the truth. Oh hell to the freaking no. 
After lying to my face more than once and then telling the truth still counts as a lie doesn't it?? He said it didn't but the thing is if I NEVER press on. If I NEVER actually double check anything he says then I NEVER find out the truth.. 
everyone says I can do better but I want him, well at least the person I thought he was..
Im from a very protective family and he was the first person I've ever talked about or introduced to my family 
I'm emotionally drained and depressed because I cannot understand why I let this get so over my head so quickly..
I hate always having to second guess myself and having no trust. 
I told him I don't want to be in a relationship like that and he constantly just says he loves me, but there's no way I can trust him or anything he says 
He tries to patch things up at times but a lie always follows shortly..
So, How many chances are too many chances..?