There are two heart attitudes that will rob a person of power in prayer - guilt and bitterness. Guilt is a result of our doing wrong, while bitterness normally creeps into our hearts when we've been wronged by others. When Jesus taught His disciples to pray, He dealt with both of these critical heart issues. There are two types of guilt - false guilt and true guilt.
False guilt comes from the accusations of Satan.
He tells us that we're no good, or that we're not worthy. You're not worthy to pray. Of course, we're not worthy enough to pray. So, we'll never be able to achieve worthiness. We can only fall at the feet of the Savior and worship Him as the worthy One.