Although I hate to say it, I think Paul has a point. For me, the 'losing mentality' comes from (or leads to) not being able to keep possession when in winning positions, even (and more especially) when playing against against bad teams.
Although we have gotten better at this in recent years, even Ars*nal seem to be better than us/more comfortable at keeping possession. That's the way it appears to me anyhow. The only reason we have been able to come close to competing (losing out last year of course) is becasue Ars*nal don't have some of the impact players that we have had.
It's this lack of confidence in keeping the ball that the England national team has always lacked - a lack of confidence in their own technical ability. This confidence comes from knowing that your club/country has pedigree I think. Tottenham, like England, although they had great success in the 60s, haven't set a footballing standard for a long time.
And whereas other clubs have been building legacies since the EPL began (Ars*nal, Chelsea, Man Utd and Man City), Tottenham have been treading water until this summer.