Trust issues


I have been with my partner for a year and a half now but we have been friends for a really long time before that. We do well in every aspect of our relationship except there is one thing that I don’t like and that is him smoking too much pot. We came to a compromise where he would smoke less and things were going great. Until I read his phone and realised he has just been lying to me about getting high instead. I confronted him about this and he has continued to do it over the last few months or so saying he is sorry and stupid every time. This has created some serious trust issues on my behalf when we had a good compromise but now I was started to resent him smoking more and more as it was the reason behind his lying. I finally regained his trust after him doing really well for about 2 months now being honest and generally just more relationship orientated to find out tonight that he has lied again :( about the same thing. My hope has been shattered because we were going so well! And communicating for him to jeopardise this and put us back to square one.. anyway just wanting to get some thoughts on this.. how many times can a partner lie and say they are sorry for the same issue to pop up again.. any suggestions on what I should be doing or if I have gone about this the right way. Feeling abit lost and clouded by my love for him.