Stay or leave?

I just don't know what to do... I've been with my boyfriend to two years now but I feel like lately everything has been crumbling down to pieces. He drinks now every day and when he does he treats me like crap. The reason why I don't leave is because he's the first man I kissed, the man I lost my virginity to, my first boyfriend to live with, my first everything. I cry just thinking about leaving but then at the same time I feel like I still have a whole path ahead of me with continuing to work in the medical field and pursue a nursing career. I just feel like I'm lost. Any thoughts?