Granted, all of that is true, though it was Soult coming from the south that forced Wellington's retreat. Psychologically, that is partially true as well, though the shakiness of the French regime was something everyone (except Napoleon,until belatedly) realized for some time. It is often overlooked just how much Spanish resistance in various ways and quarters contributed to this French failure in the long term. No doubt though that the British presence under Wellington substantially accelerated ultimate French failure as well.
However, Soult did eventually bestir himself and the Northern French Armies recovered quicker than expected - bit of hard luck on Wellington really.
The French had such a huge numerical advantage in Spain that whenever they combined they would force Wellington to retreat.
However whenever this happened the Spanish guerrillas swiftly returned to the uncovered "pacified regions". It took both the regular British troops and Spanish guerrillas wear down and ultimately defeat the French. I agree most British books do not give insufficient credit to Spanish guerrillas.