I used to have that problem as well... when I got back into otb I scored an excellent 2/7 in chicago, and gained a single rating point. In NCC, I scored somewhere around 3/5, except I was top seed in my section.
There isn't really a way out of this. Practice makes a habit, so you'll just have to... keep playing long games. Like, at least 45min. 10min games are good, but I'm finding myself in serious time pressure at around move 25 (<3min, opponent with 8).
What I did was I just... tried to find the best move. I would see a line that's pretty good, and I would start to make a move, when I tell myself, "what if there's a better move? I have plenty of time. I can spare the time. Nobody's in a hurry." and I would talk to myself and coax myself into spending more time looking at alternatives.
Other times, I would say, "Hey, why don't you take a look at each piece and compare them? You shouldn't make the wrong positional evaluation." Other times I spend somewhere around 10-15 minutes on a single move out of the opening - since once you get out of theory, you have to know the right plans. It doesn't necessarily have to be the best plan, but you have to go and stick with it.
Really, all I do is treat myself as another person.
I think the years of playing blitz have finally caught up to me... I think this is currently where I make mosts of my mistakes.
The issue is simple, I have a position, I look at the position, calculate some variations and play an inaccuracy anyways. The problem persists even in rapid and classical games when I have time to think.
I tried to force myself to actually take a glimpse at every valid move but I sooner or later go back into my habit. How do you fix that?