I've got every sympathy mate. Done it myself. It's bad enough when you realise before the kick off but when you think you've won and go back to check it's even more of a sickener.
I had one the other week, can't remember if it was on Coral or Skybet but one of them has a confirm button to press after you've chosen your bet. Looks like I didn't see the bugger through and it cost me.
Had to bite my lip and resist the temptation to double my next bet to chase my "losses".
I'd suggest you do the same mate, and, as I say, you have my sympathy (which is worth precisely fuck all I know).