It sounds like you need to take some serious time to think out your next steps very carefully. I dont think cannabis has all that much to do with this. You just need to decide whether you truely want to be with her or not and its far better you decide this now than letting some relationship play out before your eyes with no input from you.

If she would actually be that devestated and unsure whether she wanted to be with you over a factor as small as smoking you have to wonder how deeply her love really is for you. Does she want you or just someone to be married to?