it's definitely hard, greensad. Please remember that it's 100% fine to reach out to your GP; that's what they are there for! (and look, you just paid your NHS surcharge!!)
I made a dumb mistake when renewing my FLR(M), too. The caseworker was actually quite sympathetic -- and if anything, I think they've grown more sympathetic in the years since. these forms are HARD and the caseworkers know it. I know it's hard, but do try and keep on with your application and cover letter; it's really, really likely that you're gonna be fine.
Why don't you post a draft of your cover letter so the board can look it over for you?