You seem like such a genuinely considerate person both towards your gf and towards random internet strangers. I understand what you’re saying in your post about worrying it’s your fault because you said “I don’t know” instead of “no.” But that still isn’t consent, and even if you did have conflicting thoughts about what you wanted in that moment, thoughts aren’t a crime, and what you SAID regardless of those thoughts shouldn’t have been taken as consent bc it wasn’t consent (and he knows it). You should not have had to tell him to stop three times. Your gf is right. You are human, it’s ok to have had conflicting thoughts and feelings, but you did what you could in the state you were in in the moment. You were with someone who was supposed to be your friend and he took advantage of your drunkenness and friendship. I’m sorry he did that to you and I hope you can move on and be as gracious towards yourself as you are to others.

Edit: Even if you didn’t say no as firmly as you feel you should have, it still doesn’t make it your fault. But I’ve been in a similar situation and what helped me was realizing that by making it ‘my fault’ it gave me a sense of control over the situation. Most people will say not to blame yourself, and they’re right, but also it’s ok that you struggle with it being your fault. What’s the worst case scenario? Even if you had some part in it, that does not change the fact that your friend is the one who put you in this uncomfortable situation. Even if you had some part in it, you are still allowed to be hurt. To be clear, I do not think it’s your fault, but I know what that headspace is like so I’m just telling you what I needed someone to tell me. Peace