I find the same thing with writing anything longer than a couple of sentences, and that's the major reason I've stuck to a laptop.
I love my phone - and I could see me wanting a tablet as well as my laptop, but I definitely want the laptop, so the tablet becomes an "If I have the spare money" item.
In the six months since I got a tablet, it's my laptop which has become superfluous. I also have a nice desktop setup, and there is now no reason for me to use the laptop. When I'm out and about, I can count on one hand the number of times I thought it would be a good idea to bring the laptop since I got the tablet, and I never actually used it. A good ergonomic desktop setup for productivity and a tablet and smartphone for mobile use is all I really need. If I regularly had to be productive in multiple locations without a desktop handy I might feel differently, but I don't.
I love my phone - and I could see me wanting a tablet as well as my laptop, but I definitely want the laptop, so the tablet becomes an "If I have the spare money" item.