IME there's a dangerous fine line between loving and understanding an alcoholic parent.
When we love someone, we accept them as they are. We admire their strengths, and we want to take care of them where they are weak.
When we understand someone, we accept their actions/choices because we can relate. That's where the danger part comes in.
As much as you believe you truly understand your stepfather, this may be where you can draw a healthy line. It's okay to understand your stepfather; it's not okay to stop there.
For example, you to your stepfather: "I understand you felt trapped/overwhelmed/depressed, and caused you to turn to drinking. I understand you had a hard life, feel neglected/abandoned/etc. I understand because I might have made the same choices during those hard times, if I had felt so trapped too."
Your follow-up to yourself: "However, I don't understand how you could stop at drinking if you knew you had a problem. Here's where I can't understand you anymore, because I'm a healthier person and I would choose to get help if my behavior was hurting someone."
I'm the daughter of an alcoholic father, and for 20 years I believed I was the only one who understood him. I wanted so bad to understand him, believing that if my "heathly" brain could understand, then my healthy brain could also figure out the answer and help him fix himself. I also wanted to do it because if he could get better, then my pain would have been well spent. I believed that I couldn't get better until he was better. I was that immeshed in my unhealthy alcoholic father.
Both your stepfather and my father are certainly good men trapped in their own prisons of booze. But believe me when I say this: if we were to go up to these men and tell them how much we've come to understand them, I bet $10000000000 their response will be the same, "Great! Finally! Let's have a drink."
That's because these men are addicted to alcohol, which means no matter what, they choose alcohol before choosing family, work, etc.
I can understand my dad. I can understand that he's gone through a lot of pain that's led him to his drunken prison. But I can't understand his choice to stay in that prison. And once I learned to respect that he's an adult and getting better/drunk is always going to be his choice, it became a lot easier to start forgiving because I accepted his choices had nothing to do with me.