What do you mean by 'Longbow?'
Is it an ELB? E.h no handle or arrow shelf/cutaway?
If so, then read on.
You've got to almost start from scratch as it's way out at the moment.
Measure the limb thickness at the centre and the tip on that weak left limb. Mark the bow every 6", work it out roughly how the thickness should reduce in even steps from the grip to the tip (DONT take anything off from the hinged area, just taper the rest of the limb to match that).
The limb thickness usually reduces by about about 1.7 - 2mm every 6" on a ELB as a start point to get you on the tiller.
Check that left limb every 6" and you'll probably find the taper isn't even and there are a few high spots and low spots.
Even it out with a rasp etc, blending it to a reasonably even taper.
Make the right limb match it for thickness taper.
The put a shorter 'long' string on it... A 'long' string should be only just long enought to get on without flexing the bow.
The string you have on at the moment is waaaay too long.
Having done that, pull it back a bit to check the bend is a bit better than it is now and post another pic.
Del
(It's a bit like the Irishman's reply when asked how to get to Dublin...
'Ah, If I was you I wouldn't start from here'
)