Your letter is so clear and the clarity makes it painful to read. Sadly, everything you have said in the letter makes so much sense with regard to your concern about future relationships. But the good news is there are many teenagers who have witnessed something like this.
You might want to check out Alateen. This organization is free and operates groups that are likely to be near you. It would be important for you to go and hear from others about what it is like to grow up in an alcoholic home.
I think it would also be good for you to get counseling but you may want to ask the guidance counselor at school rather than your parents for help in this matter. Alateen is your best bet for now.