King Harold's force was made up of:

King Harold's army was not the same army that won the Battle of Stamford Bridge.  This was a fantastic victory, but some of the best Saxon soldiers had been killed or injured.

When King Harold had marched north to fight Harold Hardrada many soliders had joined the army on the way.  As King Harold marched south the fight Duke William, these soliders would have gone home.  During the 250 mile march south others joined the army, and these made up for those who dropped out on the way.

King Harold was advised to wait in London before fighting Duke William.  Harold's army would have grown much larger, and his men would have had time to rest.

So, the Saxon force that fought at the Battle of Hastings was different from the one that won the Battle of Stamford Bridge.  They were still a tremendous fighting unit though, and one that would never give in easily!