The system we have here is immensely complicated: a ball of protons and neutrons surrounded by 29 electrons, each interacting with the other and with the nucleus, according to the laws of quantum mechanics. A proper explanation is the kind of thing you need to take a theoretical chemistry degree to get to grips with.
But perhaps I can give you some intuition for the situation.
When you bring the electron near to neutral copper, there is an attraction from the nucleus, with its +29 charge, as well as a repulsion from the 29 electrons. It turns out that on balance, the protons 'win' $^1$. That is to say, if you placed an electron next to an atom of copper in a vacuum, the electron would become associated with the atom. However, it would only be associated loosely, because there's all that repulsion from the electrons. If you placed this system next to, say, a Na$^+$ ion, the electron would immediately jump over to the sodium where there is an excess of protons, and hence a much stronger overall attraction. The system is unstable, as you say, and the reason is that there is a good deal of repulsion from the 29 electrons.
Now suppose we had a copper ion, Cu$^+$, and we placed an electron next to it (in a vacuum, as before). In this case, there is an attraction from the nucleus, with its +29 charge, as well as a repulsion from the 28 electrons. Now, the fact that there is one fewer electron than before is crucial, because it means the net attraction on the loose electron is much higher, and as such it's going to be very happy to associate with the copper ion to form neutral copper. The system is fairly stable, and the reason is that whilst there is a repulsion on the electron from the other 28 electrons, the 29 protons safely outweigh it.
$^1$ The reason the forces don't exactly cancel out is because the electrons and the protons are in different places. For instance, some of the electrons are going to be on the other side of the nucleus to the loose electron, and because the electric force gets weaker with distance, this means that they are going to exert less force on our electron than the protons in the nucleus. It's not at all trivial to see what the net effect will be (after all, there are also some electrons that are nearer to our loose electron than the nucleus is) but if you do the calculations, you will find that there is an overall attraction, as I said.