Psychology is one of the key factors in football; there is no doubt about that. Regardless of strategies and tactics, footballers must be in a healthy psychological condition to play the match. That is why playing away games are harder, although there is nothing physically different from a home game. Being away from your usual playground and playing against thousands of people chanting for your opponent creates a great psychological impact. Nevertheless, psychology is only one of the key factors among others like physical strength, individual talent and most importantly, tactical superiority. We can make a good assessment of a team's performance by considering all four factors together.
The reason why I stressed the importance of factors other than psychology is that the English press linked the magnificent comebacks of Liverpool and Tottenham directly to psychological factors such as belief and determination. Of course, coming back from a 3-goal defeat requires a great amount of belief and determination, but it is not enough. In other words, a necessary condition for making comebacks is to believe in oneself, but that is not enough. You have to achieve superiority in the sum of all four fields that I mentioned above.
Take Barcelona vs. Liverpool for example: The first match was a great example of how tactical superiority could suppress physical superiority. Liverpool were obviously the side that had physical superiority, but the game they played forced them to cover almost the entire field for 90 minutes, which left them exhausted in the second half. Barcelona, on the other hand, brilliantly used their great possession system to drain Liverpool's energy to leave them vulnerable to Leo Messi's lethal runs. Thus, although the individual talents were roughly equal on both sides and psychological factors did not have a great impact, having tactical superiority made Barca overall superior to physically superior Liverpool.
In the second match, the story changed dramatically. Liverpool narrowed the distance they had to cover by pushing their defense more in front and Barca helped them to do so by sitting on their half and relying on counter-attacks. This allowed the already physically superior Liverpool to achieve tactical superiority, enabling them to beat their opponent with four goals and reach the Champions League final.
However, if Barca had not played so defensively and had not relied on their 3-0 lead so much, they could have easily scored one or two goals, which would have brought them to the final. Of course, Liverpool's belief in themselves was one of the key factors in this comeback, but what made the comeback possible was the combination of psychological, physical, individual and tactical factors. The same story also happened in Ajax vs. Tottenham, as the young Dutch team stopped attacking in the second half of the second leg, fearing a comeback could also happen in this game. Nevertheless, it was Ajax's decision to sit back, allowing Tottenham to finally find space and time in Ajax's half. Thus, through fear, a psychological factor, they lost their tactical superiority and gave Tottenham a chance to make a comeback. In addition, Lucas Moura's deadly finishing as opposed to Ajax players missing lots of opportunities made an individual difference.
The moral of the story is simple: Do not overlook other crucial factors when psychological factors are so evident. Without those an analysis will always be incomplete and misleading.