Disclaimer: I've been diagnosed with ADHD.

difficulty concentrating

This XKCD is a fair visualization of what difficulty concentrating feels like. I can be doing an activity (even a pleasurable one), but I get a lot of other stimuli coming in that link to different activities that also need doing or would also be fun or pleasurable. Or while doing an activity or trying to think about one specific thing, my mind jumps to other (often related) topics and this has a tendency to escalate. Think about the way people describe going to tv-tropes. You start out reading about the film you just saw, and before you know it, you're browser is filled up with dozens of tabs (all of which have links that you'll probably also click).

akrasia

Akrasia feels, to me, a lot like inertia. Sometimes in a very physical way. It's a feeling of "being stuck" and often translates to physically being stuck, without anything specific holding you physically in place. It's like the space between thinking "Doing X would be a good idea right about now" and actually doing X is a steep, uphill climb.