This is going to sound a little silly, but stick with me: Waterloo was only a catastrophic loss for Napoleon in the sense that it ended his military ambitions. What I mean by that is that prior to the Waterloo Campaign, Napoleon had suffered a string of reversals (the largest in scale and greatest in prominence probably being his misadventures in Russia) that had lead to France's defeat and his ouster and exile. Even given the success that he had experienced for much of his career prior to these events, the sheer demographic weight of fighting the rest of Europe for so long had begun to take its toll on France's economy and population. The actual battle wasn't such a bad loss that Napoleon couldn't have won, but it's a matter of debate whether he could have continued fighting for much longer if he had.

My very first post to AskHistorians goes into more detail about the prevailing political climate that allowed Napoleon to return from exile, but the summary here is that one way to understand the Hundred Days and the Waterloo Campaign (though certainly not the only way) was as an already defeated Napoleon making one last go of beating the allies and retaining (or recapturing, really) power. Napoleon returned opportunistically to Paris, and his aim in the field was to defeat his opponents before they'd properly mobilized; barring that he would defeat them individually before they could concentrate in such numbers that he was overwhelmed. This is not what happened, obviously, and when Napoleon returned to Paris with his "best shot" strategy having failed he was asked to leave soon after by the civil authorities.

I can furnish a reading list if you want to know more, but honestly you can just as easily plunder the citations of the post I linked above.

tl;dr The Battle of Waterloo itself was actually pretty closely fought, but Napoleon was already living on borrowed time and couldn't sustain the loss.

edit - I a word.

You have to look at the entire Waterloo Campaign to really understand why Napoleon lost at Waterloo. Napoleon's plan was to defeat the coalition armies before they could combine. He did this almost perfectly the day before Waterloo at the battles of Ligny and Quatre Bras.

He stole a march on the Anglo-Allies and Prussians which had them rushing to consolidate their forces. The Prussians decided to fight a defensive battle at Ligny hoping they could hold out until the Anglo-Allies joined them from the west. However, Marshal Ney(outnumbered almost 2-1) stopped the Anglo-Allies at the crossroad village of Quatre Bras. The Prussians weren't able to stand against the French alone and retreated.

One of the major failures of Napoleon during the Waterloo Campaign was not actually destroying the Prussian Army. They were allowed to retire in relatively good order and Napoleon didn't order a timely pursuit. Another failure of the campaign falls on Marshal d'Erlon. During the battles Napoleon ordered him to take his corps and assist at Ligny. So he began marching to the north-north east. However, he then received ordered from Ney, who was his direct commander, to assist at Quatre Bras. So, he turned his corps around and marched south west. d'Erlon's corps of 15,000 didn't get to Quatre Bras in time and didn't participate in either battle. 15,000 men could have helped crush the Prussians or soundly defeat the Anglo-Allies.

Now for the most famous of mistakes during the campaign, the detachment of Grouchy with 30,000 to pursue the Prussians. Grouchy was unable to stop the Prussians from joining the Anglo-Allies at Waterloo. He fought against the Prussian rear-guard at Wavre, but this battle was pointless. Some blame him for not being flexible and going straight to Waterloo to reinforce Napoleon, others blame Napoleon for not issuing clear order to him. These were his orders, The Emperor has charged me to inform you that at this moment, His Majesty is going to attack the English army which has taken positions at Waterloo, in front of the forest of Soignes. Thus, His Majesty wishes that you direct your movements on Wavre so as to move closer to us, to link your operations and establish communication, pushing before you the Prussian army corps which has taken this direction and might have stopped in Wavre, where you are to arrive as soon as possible. He was also told by a General Gerard the famous lines of,"March to the sound of the guns."

Next, the weather played a large role as well. It rained extremely hard to night before Waterloo which turned dirt paths and fields into muddy morasses. Napoleon wanted to begin his attack on the Anglo-Allies early in the morning, but the fields prevented the effective movement of guns, infantry, and cavalry. His attack was delayed by hours, which allowed for the Prussians to move onto his rear and right flank. Prussian Major-General Baron von Muffling recalled, "The Duke was most desirous of our arrival and had repeatedly declared that this was the last moment, and if we did not arrive soon, he would be compelled to retreat." Had Napoleon been able to attack earlier the Anglo-Allies might not have been able to hold out from the Prussians. The Battle of Waterloo might have turned into a victory for Napoleon just like Ligny.

Finally, once the Prussians did arrive they put a ton of pressure on Napoleon's rear and right-flank. Napoleon had started the day with an infantry reserve of 36 infantry battalions ...Keeping Plancenoit cost him 25 of them... In the event only 8 battalions of fresh troops were committed to the last attack on Wellington's ridge, of these only 5 were in the front line."

-Mark Adkin, Waterloo Companion, The: The Complete Guide to History's Most Famous Land Battle

As you can see, a lot of things went wrong for the French during the Waterloo Campaign. As much as Wellington liked the claim the victory solely for himself and the British, it was more of a culmination of errors and bad luck on the French side.