I think I’m failing my Christian values what now?

I’m an 18 year old Christian (Seventh day Adventist), and the girl I’m seeing is also 18. We’ve been talking for about a month, and she’s waiting for me to officially ask her to be my girlfriend.
From the beginning, I told myself that I wanted to lead her closer to Christ. I didn’t want our relationship to be based on lust or sexual temptation. But recently, when we were alone in the car, we ended up doing sexual things together ). It wasn’t something forced. We both participated. Afterward I realized my actions didn’t match the values I said I wanted to live by.
Now I’m struggling with what to do. I genuinely care about her. I’m very attracted to her, and I do want to be with her. But I also feel like I failed to live out what I believed. I’m not trying to blame her because we both made the choice.
I’m wondering if I should talk to her before I ask her to be my girlfriend. Part of me feels like I should tell her that I want us to have healthier boundaries if we’re going to pursue a relationship. Another part of me worries that bringing this up will make her think I regret her or that I’m judging her, which isn’t true.
For those of you who are Christians, especially if you’ve been in a similar situation, what would you do? Would you have this conversation before making the relationship official? How would you bring it up without making her feel blamed or rejected?