Your scouting is good, but the east wing, the French, Boulognnais and Manceaux for the most part (with some Normans), are not as careful. And it is only when your outriders bring news of king Harold's rapid approach, that you realize your danger: he has force-marched southward, after defeating the Norse invaders up around York, and is intent on surprising you before you can get your scattered army back together. In desperation you send messengers to where you last knew your western and eastern wings to be: they are to fall back toward Hastings until you are all regrouped.
But Harold's information is accurate, and he descends upon the eastern wing before they can catch up to where you and your Normans are waiting. The eastern wing is annihilated by the much larger English army. You are cut off from the coast and must fight a battle against very superior numbers. As the English warriors man for man are a match for yours, the outcome is almost certainly defeat for you. Toss a coin three times and win the toss twice: you are able to drive off the English sufficiently to regain the coast and board your fleet. If you won all three tosses, you also killed Harold in the bargain: but if you lost two tosses then you also lost the battle and are a prisoner; if you lost all three tosses you are dead.
If you won the battle, it does not follow that you can win England: after all, your army is cut down drastically in size. If Harold is dead, his brothers will take over command anyway, and pursue you with a reinforced army: they have more troops to draw on, you do not. If you escape back to Normandy, there is no second chance for you. History will remember you as William the (rash) Bastard.