Hi there, first things first don't panic! The advice 111 have given you is pretty much it, drink some lucozade and also eat something starchy, like bread, or weetabix, something like that. Keep hypo treatments nearby, and definitely keep testing. If you can stay at home, that's great, and try not to be too active. Is anyone with you? Just keep testing, and keep eating if you need too. I know it's not nice if you end up going too high, but it is better than having a hypo.
Try not to stress too much! We've all got it wrong before and, speaking as someone who has overdosed in a completely different context, it's not a massive massive overdose, so you can probably control it without having too bad a hypo. Keep testing! Did you eat the meal you were dosing for?