Instead of having a flexi day have you thought of trying alternatives for your biccy, eg saving hexb & having an Alpen light, or perhaps a chocolately/desert tyep yogurt. What else do you find yourself eating which is high in syns?
If flexi syns were as simple as 'eating what you want & then making up later in the week' you may not be able to stick to that anyway with your 'i've blown it' thoughts.
I'd agree with the others flexi synning is best left for special occassions.
Have you thought of finding low syn biscuits, like pink n whites instead of what you normally have, could you limit yourself to say 3/4 per day?
Is there a pattern to when you go over your syns, say at the weekend, how many syns do you have when you go over?
I know I've asked lots of questions, but I think if we try we may be able to find something which will fit in with your lifestyle & something you can stick to.