Hello and welcome. My heart hurts for you.
I think that your therapist is saying not to repress the feelings in that you need a face to face network of support. I am thinking that the stress of hiding this from your friends must be very intense. I hid my Xhusbands drinking for a long time. Low and behold, most knew anyways! The day I stopped hiding it, and reached out with love, is the day my life truly did change.
I was honest with friends and family, I kept the focus on me, that I need a support system to go to in times of need. After all, isn't that what friends and family are for? I encourage you to reach out and gain that support network for you as well.
As far as what to do when he is drinking, you are 19. Are you involved with many things socially? If not, let's think of something for you to focus on that can be a positive in your life. What do you like to do? Create something to take the focus off of your dad and his drinking.
Lastly, my daughter's therapist encouraged her to write a letter to her dad. It was kind and loving, but did explain how his drinking affects her, and how it makes her feel. It was very good for her to do this. It took a long time to build the courage to write it and give it to him, but it did help her.
Keep posting here at SR. This is a place of great support and encouragement. We will be here for you.