I think he was a very sensitive man, he was very perceptive and that combined with his childhood shaped him into a man with an inferiority/superiority complex. Considering he had these issues and still achieved some great things shows that he has alot of heart so it depends on how you define 'mentally weak'. I wouldn't say that he is.
I do not think he was weak as much as he was so dominant that he was never hit in his prime. Then he starts to get hit and he is not used to it. Once he was beaten by Douglas, he actually got in wars more and did ok. Some of those punches by Razor Ruddock were very hard and Mike took it all.