I cannot exactly diagnose your problem without a video, so this is based on how you explained it.
What it sounds like is that you are using incorrect technique. The back extension roll done correctly requires a roll then shrug, not a throw your head back and reach for the ground then bend and push.
Now for a couple of things that could help.
First one is a drill I use for my girls. You start in the pike position, then initiate a backwards roll to push up position, not to handstand, this is to teach correct form. Just focus on keeping your arms straight and shrugging your shoulders. You can progressively move this to a handstand, but don't try to go bigger until your form is flawless. If this is too hard, start on a wedge.
Slider conditioning, shrugging against the wall is good to build strength.
If the problem is that the stand sit down is messing you up, try rolling back to a candle stick and shrugging, but not shooting your toes up, then adding the roll. Make sure you are sitting it down but not pushing back.
I can't really help you with that unless I see a video, because likely the shrug is not the root problem but a symptom. Maybe someone else will be able to help with drills. If you want to, you could post/message me a video for further advice. Sorry