Of course there's such a thing as being out of someone's league. If a girl goes for powerful guys with big wallets then you're not going to have much luck with your job as a cleaner. That's life.
You have to have something to offer to the opposite sex to be in their league. If you don't have that something then you're out of their league. The best mindset to have though, as someone mentioned, and this comes with confidence, is to believe that you do have something to offer the opposite sex: say you don't have as great looks as a lot guys or say you don't have that much money as most guys but if you're confident, you believe that those guys have nothing over you and that you have just as much to offer.
However, you have to accept that quite often, what you have on offer won't be too all girls' tastes (this can be interpreted as being out of their league but doesn't have to be).