an exit

I was supposed to spend this weekend with L. It's 4:11 PM on Saturday and I'm home now. Last night there was an incident -- an agreement was broken; a bond of trust was cracked.
L smokes pot (as do her roommates) but I do not. I quit smoking marijuana several years ago. I had reluctantly tolerated her pot smoking over the past two months, but recently I had expressed my displeasure about her smoking habit. I don't enjoy being in the presence of those smoking pot; I feel like the designated driver at a party full of drunks. She didn't agree to quit; however, she agreed to cease smoking around me and cut back her weekly consumption.
Last night I was with L in her bedroom watching David Letterman interview Ira Glass when she exited the room and joined one of her roommates in the living room. After a few seconds I peered into the living room and saw her smoking pot with her roomie. She coughed into her arm (I'm assuming to muffle the sound and avoid arousing my suspicion). I was dumbfounded she would break our agreement practically under my nose. She returned to her bedroom, and I confronted her about what I had witnessed. I don't recall her response -- that doesn't matter. We had a mutual agreement and she broke it. Moments later, as we lay in bed, I was very upset and said I would probably leave Saturday. I awoke the following afternoon, and my feelings hadn't changed. I remained agitated and at one point she said "just fucking leave"; however, I don't believe she meant those words because she later pleaded me to stay, but the "just fucking leave" remark only added fuel to my fire and another wound on my heart. I gathered my things and left without a "goodbye."
Despite what this post might suggest L is the only girl I've ever met who I believe truly loves me. I feel so undeserving of her overwhelming kindness.
Two challenges face our relationship: distance and, now, the pot issue. We're separated by 90 minutes which means weekends are our only times to spend together; weekends are also the best time for me to write, and if I seriously wish to pursue my dream I have to commit to more writing time. Regarding the pot issue, I must confess I don't see how or why this is a problem. If one is truly committed to his or her relationship why would he or she let pot risk destroying that bond?
Tomorrow will be our four month anniversary -- wonderful timing.

mc