Yea this is a very big issue in Islam. Actually a pillar of belief is indeed the belief of pre-destination or qadr.
As you've stated before our birth EVERYTHING is already determined. How much we'll make, how many kids we'll have, if any, where we'll live, when we'll die, how we'll look, good or bad, etc....
But that doesn't mean we don't have the chose to do these actions. Allah gave us free-will and this is so clear even before man. It shows up first when Allah told the angels to bow before Adam;
Chapter 18, verse 50
Behold! We said to the angels, "Bow down to Adam": they bowed down except Iblis. He was one of the Jinns, and he broke the Command of his Lord. Will you then take him and his progeny as protectors rather then Me? And they are enemies to you! Evil would be the exchange for the wrong-doers!
So you see, when Allah made the jinns, he gave them free-will, ie Iblis disobeying Allah because of his pride in himself. (Angels, no free will, must always obey). But He knew that Iblis would disobey and He knew that this would happen.
Chapter 2 verses 29-39
29. It is He Who has created for you all things that are on earth; moreover His design comprehends the heavens, for He gave order and perfection to the seven firmaments; and of all things He has perfect knowledge.
30. Behold, your Lord said to the angels: "I will created a vicegerent on earth." They said: "Will You place therein one who will make mischief therein and shed blood? - while we do celebrate Your praises and glorify Your holy (name)? "He said: "I know what you do not know."
31. And He taught Adam the nature of all things; then He placed them before the angels, and said: "Tell Me the nature of these if you are right.""
32. They said: "Glory to You: of knowledge we have none, save what You have taught us; in truth it is You Who are perfect in knowledge and wisdom."
33. He said: "O Adam! tell them their natures." When he had told them, Allah said: "Did I not tell you that I know the secrets of heaven and earth, and I know what you reveal and what you conceal?"
34. And behold, We said to the angels: "Bow down to Adam:" and they bowed down: no so Iblis: he refused and was haughty: he was of those who reject Faith.
35. We said; "O Adam! you and your wife dwell in the Garden; and eat of the bountiful thing therein as (where and when) you will but do not approach this tree, or you run into harm and transgression."
36. Then did Satan make them slip from the (Garden), and get them out of the state in which they had been. We said: "Get you down, all, with enmity between yourselves. On earth will be your dwelling-place and your means of livelihood for a time."
37. Then Adam learnt from his Lord words of inspiration, and his Lord turned toward him; for He is Oft-Returning, Most Merciful.
38. We said: "Get you down all from here; and if, as is sure, there comes to you guidance from Me, whosoever follows My guidance, on them shall be no fear, nor shall they grieve.
39. "But those who reject Faith and belie Our Signs, they shall be Companions of the Fire; they shall abide therein."
So you see, Allah said that He would create vicegerents on earth! and low and behold! Although Adam was in the Garden Allah knew what would happen, and Adam had it written in his destiny but he also had the FREE WILL. Yea Satan persuaded them, but they could have chosen not to listen. ++ also another good point to mention here is that it says that satan, or iblis, was of those who reject faith. Although he was right there, worshiping Allah with the angels and in heaven! Though because of this disobedience we see clearly now that he was not of those with Faith. A lesson for us to remember...++