The final stretch is not the same as the first 90%. You don't know what you don't know at the start, you don't have formed ideas or boundaries to butt up against and so progress is easy. As you create, you have gained more knowledge and now have to consider you surroundings, the problem space becomes less open. The closer you get to completion the less space to work in, you likely have greater taste, ideas which are good enough for the first 90% are no longer good enough. So better taste and far less acceptable solutions due to increased dependencies. You also can see parts which were previously acceptable as now not so.

To top it off you have the emotional and ego side at play near the finish line. Is this good enough? This could be done better, etc.

I think that mixture of better taste, more dependent parts and ego make the last part the hardest. I also feel that the finish line being close isn't a strong of a motivator as ego is a demotivator. Whereas, at the start ego has no effect as you don't know anything, you can't be mad because you're new, it's all one big playground.