Elbow Bridge (advice?)

This was my first attempt (well, second, the first one I thought I would die, haha). The athlete in the technique video has beautifully straight legs and I feel there is no way I can straighten them more. Shall I keep practicing like this and get straighter with time (hopefully) or shall I try to elevate myself like in the TBS series? Any additional exercises I could do to help my upper back and neck to work with me on this one? (I own all stretches and all foundations and handstands 1+2). Thank you for your input.  

PS: Even when I was 16 years old, my spine didn't bend backwards… 

I would try elevating the feet. Also pre-stretching the pecs and lats and abs, will go a long way to squeeze out some extra thoracic extension.  I've got pretty decent mobility in my chest and lats, but its my tighten/short abs that kill my bridge work...

@Julian Aldag, thank you, these are great tips. I did not do any extra stretching of those areas before. That makes a lot of sense. My abs are tight, my hip-flexors even tighter. Chest and lats getting better than they used to be, but still need more work. I'll do that on Thursday, and see if I get a better thoracic extension. Even will try elevated feet…