The heart in its journey towards Allah is like a bird; its head is love, and its two wings are hope & fear. When the head and the two wings are sound and healthy, then the bird will fly well. If the head of the bird is cut off, it immediately dies, and when either or both wings are deficient, the bird cannot fly properly and it may become the victim of a hunter that passes by. - Ibn Al-Qayyim Al- Jawziyyah
One of the great guiding principles of Islam is that the believer should tread a path between fear and hope. He should not take it for granted that he will enter Paradise, because this will make him complacent, and he does not know in what state he will die. Nor should he assume that he is going to Hell, because this is despairing of the mercy of Allaah, which is forbidden. So the believer does righteous deeds, and hopes that Allaah will reward him for them, and he avoids evil deeds out of fear of the punishment of Allaah. If he commits a sin, he repents in order to gain forgiveness and protect himself from the punishment of Hell. Allaah forgives all sins and accepts the repentance of those who repent. If a believer fears that the good deeds he has sent on before him are not enough, as you suggest, then he will increase his efforts, in fear and hope. No matter how many righteous deeds he has sent on before him, he cannot rely on them and take them for granted, or else he will be doomed. He keeps striving and hoping for reward, and at the same time he fears lest his deeds be contaminated with any element of showing off, self-admiration, or anything that will lead to them being rejected by Allaah. Allaah describes the believers :
What is the matter with you, [that you fear not Allah (His punishment), and] you hope not for reward (from Allah or you believe not in His Oneness). (Surah Nuh verse 13)
So the believer keeps on striving, fearing and hoping, until he meets his Lord, believing in Tawheed (Divine Unity) and doing righteous deeds, and earns the pleasure of his Lord and Paradise. If you think about the matter, you will realize that these are the right motives for action, and that righteousness cannot be achieved in this life in any other way.
Anas reported that the Prophet(pbuh) entered upon a young boy who was dying. The Prophet (pbuh) asked: "How are you?" The boy replied: "O Messenger of Allah, I am in-between hoping in Allah and fearing for my sins." The Prophet (pbuh) said: "The like of these two qualities do not unite in the heart of a servant except that Allah gives him what he hopes for and protects him from what he feared." (At-Tirmidhi)
Taken from www.islamqa.com