Help with Overly Coarse Nettle Fibre?

Hello everyone!  I have been really interested in trying hand-spinning for the past few months and have been looking for fibres to spin.  Sadly, I was not able to plant flax this spring.  However, I read that stinging nettle fibre can be processed into a flax-like fibre which can be spun in the same manner.  Upon hearing this, I visited a large, abandoned field where there are tons of nettles and harvested a bundle.  I then dried them in my greenhouse and retted them in my lawn for about two weeks (until the stems started to become dark in colour).  Once they appeared sufficiently retted, I dried them again and broke them.

However, I found that a thin, coarse black matter still clings to, and binds, the outer surface of the fibre.  The woody bits on the inside were easily removed by breaking, but this seems more stubborn.  The first attached photo shows nettle fibres on the left (the top fibre just after breaking, the middle after I picked through it, and the bottom is some cordage I spun from it), on the right is some much finer flax fibre which I received from a local farm museum for comparison.  The second is a close-up of the picked fibre, showing that it does indeed have very fine fibres, but that many are bound up with that tough outer-layer.

I am unsure exactly how to remedy this problem.  Perhaps I didn't ret it for long enough?  Or maybe stinging nettles aren't suited to dew retting and I should try submersion retting?  I appeal to people more knowledgeable on these matters than I am, in hopes of receiving some guidance.  Thank you all for reading, and sorry for any formatting problems; I am new to posting.

This lady explains to scrape the nettles to get the fiber out.  I have not done  this though I am hoping this will be beneficial to you.

She is prepping to spin and you are prepping to braid though the technique to soften the fiber might be the same.