This is not your fault. None of this is your fault. We are flying blind when alcoholism enters our lives, and we are extremely vulnerable to an alcoholic when we fear losing that person for good.
We try to outmaneuver the addiction (which is a disease of the brain and beyond our control), we try to bargain with the alcoholic (only 3 drinks a day, only on weekends, only beer, only when we go out to dinner, etc), we cry, beg, plead, threaten, lecture, and punish with silence or with shouts....but nothing we do cures the alcoholic or the addiction.
If you have a desire to marry this woman one day, then the only chance you have for a healthy relationship some day --far out in the future-- is if you both go into recovery.
Generally the sequence of things is the loved one goes into recovery first, changes his enabling/codependent behaviors toward the alcoholic, and then one of two things happens:
1. the alcoholic experiences the full consequences of her addiction, and those consequences could be a break-up, a separation, a DUI, the loss of a job, a health crisis, or other kinds of serious crises. The alcoholic eventually reaches a point of dark hopelessness and enters recovery. The relationship is resumed after a long period of stable sobriety and personal growth by both individuals. (one would hope).
or 2. The alcoholic continues drinking in spite of all negative consequences, and the partner has such low self-worth and is so isolated and controlled by the alcoholic that he stays with her no matter how poorly he is treated nor how many of his most basic needs are being unmet. In this scenario it is not uncommon for the relationship to eventually end anyway, as alcoholics are often unfaithful to their partners, having lowered inhibitions when intoxicated, or resentments during the dry periods.
If you would like to give #1 a go, then you will do well to start attending Al-Anon faithfully. If your relationship is like most with an alcoholic, you will break up, make up, have a pink cloud period, break up again, make up again, another pink cloud, and this could go on for years. But if you go to Al-Anon and work on your own issues as they relate to your own values, your own principles and goals, and your own recovery from being codependent in a relationship of addiction, you will, over time, become clearer, stronger, and able to sail your own ship no matter what she is doing.