An autistic expressed exasperation at how hard it was to do something today that had been relatively easy to do at another time. I responded:
For me, there's often two overlapping things going on (that make it hard or impossible for me to do "simple" "easy" things). One is that various things take different kinds (and, of course, amounts) of energy. I can have an adequate amount of one kind of energy at the same time as I totally lack another kind of energy. Which kinds (and amounts) of energy I have at any one time determines what I can (and can't) do at that time, and also how difficult each thing is for me. Thus the phenomenon where I am unable to accomplish "simple task A" for a week/month/year, and then suddenly one day I can do it. That's the day when, no doubt for elaborate and long-term reasons, I finally have accumulated a sufficient amount of the kind of energy required for task A.
Sometimes having enough of the specific kind of energy required for a task is not sufficient if there are things that must be done in order to arrive at the beginning of the task. Example: I might have enough energy to endure a taxi ride but not enough (of the right kind of energy) to handle making arrangements for that ride. Last week I was able to take the bus to a doctor's appointment, but after the appointment it was much easier for me to walk home instead of taking the bus. I had the kind of energy required for a 90-minute walk, but not the kind of energy required to "handle" the uncertain stresses of a bus ride. Living through the appointment had altered my internal energy supply.
The other thing that can have a major effect on my ability to do things is...not sure what to call it. Phobias? Disinclinations? Locally heavy accumulations of disinterest expressed in the form of inertia? Each of those is different, but each is something that can trip me up by making me "unwilling" to do something I "ought" to be able to do. I put "unwilling" in quotes because, although in theory I could do the task, in practice I would be tearing myself up somehow if I tried to force myself.
Mental blocks appear in my brain sometimes. They wax and wane like tics, but they're a heck of a lot more interference in my life. Some of them last for decades. Others are more ephemeral.