Newbie <3 months into lifting. Save me from my deadlift form!

Started trying deadlifts this week (50 kg at 60 kg body weight). I only feel it in my back. No interest from my hamstrings. In my starting position I aim for a slightly blunter than 90% knee angle, shins to bar, bar behind shoulders. That’s what I’ve read (hope it’s right!). Leg press first, then straighten. Problem is, that means I lift with a horizontal back and it hurts like hell. Only way to get it more upright seems to be more knee bend, but then either my knees have to come forward far over the bar and I hit them with the bar, or my butt has to go back and down and then I literally fall backwards. It feels like my legs are too long to fit! Help!