There are two general directions from which one can come at an issue like this: the instrumental and the operational.
"Instrumental", the way I'm using it here, would have to do with aspects of the work of writing that involve the interaction of the writer with writing (and the written) through a given interface. These are, broadly speaking, external issues. They can be tweaked and poked at with varying levels of success, and when things have been malfunctioning, such tweaking usually shows results in the relatively short term.
Operational issues, though, are ones affecting what's going on inside the writer before (or during) getting anywhere near the interface. I'd say this is the kind of thing you're dealing with.
Operational writing issues can be challenging to debug...and even when their nature or causes seem clear, the the results of the debugging can take a while to implement. So before we start getting into this: with regret I have to tell you to set aside any thoughts of a quick simple fix.
Briefly: my guess—from the symptoms you've described above—is that something writing-associated may have happened to you a year or so ago, or perhaps earlier, as sometimes the effects of this kind of situation can take a while to manifest. (And that you're able to identify the time frame that clearly speaks well of your powers of observation.)
Now let's be clear: whatever this thing was that happened, it doesn't at all have to have been anything bad. In fact, it's just as likely, or maybe more, to have been something really good, a positive life change—a shift in a location or a relationship to something newer or better. But its effect in the longer run is that your relationship with your internal writing process has changed, or your perception of it (and the way it felt) has shifted. Or possibly both.
As regards analysis of the a-year-or-so-ago situation: even if you successfully identify an event that seems likely to have been involved... that identification, by itself, is unlikely to make any difference to your current situation. The shift has happened, and seeking to simply undo it by processing it somehow is likely to be a waste of your time.
The bottom line is that if I'm right about this, you and your interior writing process have for some reason started growing in a significantly new and previously unexpected direction. The reason writing doesn't feel easy or fun these days is that the new direction of growth is causing you to need to expend more energy and effort than previously—because growth does that. The sense of effortlessness that used to be there is missing now because the new, more complex level of function can't be carried on, or carried out, with the old sense of ease.
The tl;dr; Odds are strong that you're currently on your way to being a significantly better writer. You just can't do it on the same terms as you used to.
If this sounds like bad news, it's because of course you'd like things to just (after some quick useful instrumentational adjustment or other) go back to the way they were! Unfortunately, though, I think that in your case, that ship most likely sailed a year or so ago.
Your options now are somewhat limited. You get to either:
(a) Sit around where you are (developmentally speaking), writing "retrospectively" in an attempt to recall that sense of ease, and be really pissed off about this whole mess:
(b) Admit the likely context of the growth change you're edging into, and start the work of pushing through where you are at the moment to the other side.
And the only way you're going to be able to do that is by just keeping on writing on a regular basis, even though you're not satisfied with how you feel while it's happening, or what's coming out.
I'm entirely aware that this isn't going to sound at all pleasant. It sure wasn't the last couple times I went through it.
I've had two, possibly three of these—let's call them "reconfigurations", for lack of a better word. Fortunately they haven't seemed to come oftener than once every decade and a half or so. Just as well, because they're really annoying. (Especially when you're on deadline at the time.)
I mean, here you are going about your writing life and everything seems to be running smoothly, and then all of a sudden there's some kind of triggering event. (Or an unexpected internal shift that simply coincides accidentally with an external event that might or might not have had anything to do with the internal events. Even with my psych nurse hat on real tight, I haven't always been able to tell.) In the long run, the reason for the shift hasn't mattered: it's simply occurred. There's no way to go back, and nowhere to go but forward.
The only good thing to say about this kind of situation is that eventually—assuming you don't give up, or try to cheat on the increased energy required to push through to the new level—you will come out on that far side significantly stronger and tougher and better a writer than you were before everything apparently went to hell. You will also find the enjoyment again, and a renewed sense of ease. But they'll be different sensations, hooked to different and more powerful levels of function.
If I'm right about all the above, you now have ahead of you a difficult and annoying slog. And it wouldn't be kind to pretend otherwise.
My advice to you, if any or all of this rings true, is to take the writing part of it in baby steps, but regular ones. And no matter how much you may have been accustomed to write in a day, you'll be wise to scale your expectations way back while starting to deal purposefully with this. You are going to have to push through the feelings of dysfunction and unease and regularly commit words to screen or paper whether you feel good about them or not. This is not going to be easy. You are going to have to build yourself—at least temporarily—a new normal, a version and experience of regular writing that doesn't depend on you necessarily having an easy or good time of it
You are going to have to keep reminding yourself that growing pains of this kind are normal; that you're building new mental "muscle mass" and "bone structure" to support the increased stresses of the writer you're going to be when you come out on the far side of this.
Now, the nitty-gritty. How long is this going to take?
Probably wisest to assume a matter of months. But beyond that, I wouldn't have data enough to predict how long this is going to take for a different writer. What I do know is that I didn't start seeing signs of improvement or increased ease of operation until I had both realized and acknowledged what was happening to me and stopped trying to go backwards to "the way things were". Then, slowly, like the crocuses coming up in late winter, things little by little began to shift. Those first green shoots won't come up all at once. But they will come.
Also possibly helpful: watch your diet. Make sure you get enough protein: boost your intake if you can. And a good B-complex supplement wouldn't come amiss, either. Brainwork needs those vitamins in particular.
In any case: good luck on the journey. This is going to be a rough haul... but you can do it. Go well. :)