This is a pretty typical behavior that happens periodically whether it's development, marketing or many other functional areas and tasks.
I'm a marketer. When I'm working on an event or a plan or an ad or a script or whatever...often the last 10% or so is the toughest to push through. I get excited about the project and what it's supposed to achieve...I get excited to learn and prepare however is required to define the project...I get excited through the creative development process as it takes shape...I get excited to see it get close to the original vision (or however it changed through the creation process)...then for whatever reason, I just want it to be done. Sometimes I lose steam to do the last bits to complete it. Perhaps it's because those are fairly small and don't give me any type of rewarding feeling...it's just polishing, refining. Doesn't give nearly the positive buzz.
I've seen this every company I've been with, in many functional areas. Not always but enough.
So just be aware this is a very common feeling, nothing wrong with you in particular. If you feel it happening, figure out a way to push through it.
What I do if I'm stalling is very clearly articulate very specific tasks I need to do, list them out and motivate myself to get each done, thereby finishing my project. Sometimes I'll take a little break, go for a run or something else just before pushing through the effort to completion. It's good to have a fresh and rested brain. Often just a little break can make a big difference.
Best of luck!