that inner voice...

This is a subject that highly depends on the situation. When it comes to 'material' things: a work, a project, a drawing, whatever in that league of physical, material stuff, they work on differently than when the situation is about human beings and the endlessly more complex consequences. I don't know which one it is, so I'll just babble around.

I personally think it's a matter of self-esteem. I was actually going to answer 'it's a matter of common sense', but I rethought my words and I'll develop that bit further ahead. I think it's mostly about self-esteem, or rather, lack of self-esteem, and indeed, stubborness associated with it. I have that, both of those, which is why I say it, I hope it doesn't sound too harsh.

I don't think the thought of "I can't do it" is just a form of denial, or at least in the more simplistic sense. But it can be, yes. However, I really think it's all about self-esteem & stubborness by association. It makes one doubt of themselves, see many of the difficulties that'll most likely happen in the future. Lack of self esteem makes one look at those things, of all those difficulties, and just say 'no way I can do that, it'll be too much', and stubborness doesn't allow to get over it easily.

But I think it's also about common sense in some cases: a completely roughless truth. There are things that can not be accomplished by some people, or at least at some moments of their lives. In these cases, the morally-right saying of 'You can do it' is just vain. This is, as everything in life, a matter of perspective and depending on the situation obviously, and I don't know which is your situation so this is just my attempt to make my views understandable.

But it can also work as a boost. The initial impact of 'I can't do it', either brought up by lack of self-esteem or common sense, may actually work to make you decide to do it. "I can't do it? Well why the fuck not?" If one has a bad relationship with one's inner self, it may actually help in some cases, because it makes people struggle against their own opinions. The reaction becomes a confrontation and the will to fight against it and be successful arises - and when you can actually do it, it feels better than anything else.

I personally think that in situations when a "I can't do it" arise, the 2 biggest forces to overcome it are opposite but equally powerful and work out: supreme love and supreme hate. It may sound cheesy and wierd, but I personally think so. The other one, equally powerful, is absolute necessity, but that one takes away your free will. Let me try to add examples to make my point across:
- supreme love: it works through devotion to something or someone. You overcome the "I can't do it" by thinking thiings through, the motifs behind the indecision and the results that you'd want to accieve if you were successful. It can be quite working
- supreme hate: it normally demands the confrontation with oneself, and/or an exterior factor that makes one feel hatred. If you say to yourself "I can't do it" and then just claw your skin off with rage against yourself, it can make you just work everything you have to proove yourself capable, just to show that you were wrong. And when you have the thought because someone else made you insecure, the hatred may fuel you to accomplish like not much can
- absolute necessity: it just takes your free will. You HAVE to do it, regarless your opinion, self-esteem, stubborness, whatever. This is more on the human side of decisions, rather than the 'material' one I think. For example, when you find yourself in a situation where you don't have money to eat, for example, you just HAVE to do something about it. It's not about decision. It's an extreme.

In the end, like any other decision, it's a very complex process of measuring up the person's will, time, ability and consequences, everything heavily weighted by self-esteem.

Well damn, did I write a lot. I don't think I'd have if it were someone else, because I'm sure I sound wierd and annoying, but I guess I started making some insightfulness and personal analysis in the middle which is why I rambled. Sorry. I hope something in here makes sense and not too off topic.