Hi Lenore,
Just read your post and yes I can very much relate to this for me its my biggest psychological issue and I'm pretty sure its at least in part to do with Dyspraxia.
My brain seems operate in one of two modes; either complete disinterest or obsession.
In the first instance my brain works something like "Na you don't need that information
(memory dump activated), now here's what I really want you to think about", I don't mean I've forgotten stuff I mean it's just not there even though I know I'v been Involved in it.
When this is the case I find it really hard to concentrate and take in information and I have zero motivation to do anything, even when I do get my head into it the slightest thing will completely derail my train of thought (me on Pluralsight yesterday: "Oooh look a bird!, wonder if its a Crow or a Jackdaw nope it's ... CONCENTRATE damn you !! " )
In the second I just keep running things round and round in my head and tend to go "Down the rabbit hole" often horribly over thinking and coming to some weird and (or often not so) wonderful conclusion.
Its sort of a curse and a blessing for me.
I work as a software developer so if I get a problem that really interests or annoys me I will just work away at it until I've found a solution simply because my brain won't let me stop trying to work it out.
I've dug my self a bit of a hole at work for being the the guy that people turn to when they've got some bug that's been hanging round for ages that none else can sort because I obsess I tend to find angles no one else looked at, so it does have advantages.
On the other hand it also has it downsides, quite often something will just "click" in my brain and I will start to obsess about it even if I don't want to (I've found its often when I'm stressed or board).
it could be literally anything (something I've read, seen on TV or some ones said to me) normally it's something negative about myself I'll then take it, over think it and blow it out of all proportion get then very stressed and anxious.
I often wonder if its some form of OCD and have seriously thought about counselling it can be really depressing, for instance many years ago when I was struggling to find work for some reason I got it into my head that I was vastly overweight ( 6'2 and 15 1/2 stone so chubby but not huge) so I started dieting and working out obsessively dropped to like 10 stone to the point I started feeling ill, as soon as I got a job I stopped working out and regained most the weight (d'oh

)
Wow sorry that was more a brain dump /therapy session sorry

... here's points that I hope might actually help.
1.)The best way I've heard Dyspraxia described is "
A neurological impairment in the ability to organise thought and movement in the same way as a neuro typical " so I would guess most Dyspraxics will struggle with planning tasks.
In terms of tasks you know your gonna struggle doing try and break them down into smaller tasks, I just bullet point them and then see if you can spot an order in which they need to be done, quite often I struggle with this cos I can't see anything but a big "lump of stuff" so I ask a non Dyspraxic for help.
I find doing this means my mind is less likely to wander cos I know what I have to do and I can just focus on it get it done quickly as possible before I loose concentration, also it makes you feel like you've achieved something even if you only tick one thing off.
2.) Try the 20-10 method i.e 20 minutes work followed by 10 minutes break don't expect yourself to just sit down and do it till its done, even people with exceptional powers of concentration will stop absorbing information after this time period.
3.) Set yourself realistic targets, sometimes I'd sit down for eight straight hours trying to do something I didn't want to and then be really frustrated when I achieved nothing, now I try to be more realistic more like a couple of hours through the day, seems counter intuitive but I do find I often get more useful work done that way.
4.) Just walk away, whenever I'm doing something semi-important iv found its useful to actually forcibly break my concentration, I set an hour alarm on Alexa as soon as it goes off I force myself to get up and do something else, the reason I do this is two fold one because if I'm obsessing over something it forces me to break off for a bit, two if I'm struggling to do something I often find walking away for a breather helps me refocus.
Sorry that was a bit of a ramble These are just some things that help me hopefully they'll help you.