Hi.
Although you are going to see your father soon, there's no reason that you have to figure out how you feel or entirely forgive him NOW. Alcoholism is a serious disease and it will probably take a good deal of time before you, your father and your family recover and are capable of relating in a healthy way. Your father has taken steps to help himself, which is really positive, but the fact remains that he has hurt you in the past. While you love and miss your father, it isn't unusual for people with alcoholic family members to also feel isolated, angry, depressed, powerless over their environments and resentful towards the addict. Such feelings can't be expected to go away in a day and it is important that you take things slow and remember to take good care of yourself and your emotional health, first and foremost.
In terms of the actual visit, don't be afraid to leave or just step away to give yourself time if the situation gets too uncomfortable. You might also just concentrate on what you want to get out of seeing your father and trying to enjoy those aspects of the visit.
There are a number of organizations that you also might want to look into for help with dealing with a parent with a problem. They can help put you in touch with other teens in the same situation and, perhaps, you can begin to get some perspective on what you are doing through.