Bad Macro
Bad macro isn't a thug. It doesn't walk up to you, get in your face, and start shouting at you. It'll beat you up like a thug, and it'll take your lunch money like a thug, but it's not a thug. It's something more sinister, more subtle-- more deadly. Bad macro is more of a pickpocket, who waits for you to get distracted and grabs your wallet. You don't even know you've been had until he's long gone, until it's far too late to fix it.
This is what I tell my students, and I never realized it was true until I said it: Bad micro is a hot stove, and bad macro is a pot of cold water on that stove. If you touch the hot stove of your micro slipping up, you immediately pull your hand back from the heat. When your army gets FFed in half, or you get stormed before you can EMP, or banelings get all up in your business and kill your marines, you know you made the mistake, and you know it immediately.
In high-pressure situations, if you don't think, you'll stay on the ball with micro because of the immediate feedback it gives. Macro, though, is a pot of chill water on the hot stove. If you mistakenly put your hand in the pot, it will take a long time for it to get hot enough for you to notice it's hurting you-- and by then it will be too late and you will be heavily burned, for a mistake you began a while ago. Idle production isn't instantly obvious the way micro mistakes are-- it takes some time for money to build up and for your army value to fall behind.
To put it in perspective, let's take a look at what causes a reasonably skilled player's macro to fall apart. Let's say I'm sitting on 2 base, recovering from my 1 rax FE, and I scan and see a warpgate allin incoming, or maybe a 3 gate robo immortal allin. I have a small period of time before he hits the front of my nat to make supplemental bunkers-- it's going to be a close fight. I have 1 completed bunker and 2 or 3 mostly-finished ones when his push hits my front. This is when things get tricky.
In this sort of situation, without constant focus and discipline, tunnel vision sets in. I have however many marines and some scvs and I'm trying to repair and make bunkers and focus down his units, and through intense effort, micro, and force of will, I push away his army. Whether or not I hold off his 2nd wave combined with the remnants of his 1st wave remains to be seen, though-- in my determination to repair the correct bunkers and load marines, I stopped producing units for two production cycles. Whereas my opponent has gained 2 immortals and several zealots and sentries, I still have the same army I had a minute and a half ago. My macro slipped.
It takes palpable effort to rip your attention away from the battle at your front, even briefly, to tap out the hotkeys for more units (and more scvs!) or to actually move the screen and make depots. One naturally wants to watch the battle, to avoid the hot stove, but diving into the pot of water to do so only means that you won't realize you're getting burned until later. Fighting off this tunnel vision, being willing and able to rip my attention away from the push for a moment to make my next CC, to slap down more production, to call down mules and swap addons-- this feat requires determination and focus.
I suppose the first step to having slightly-less-terrible macro is to realize what exactly causes you to have terrible macro. For me, it's tunnel vision during all-ins and anything defensive. On the attack, I'm more able to remember "ok I need more units" whereas when I'm defending a base, things seem much scarier, and I'm more prone to be like "OMG IMMORTLS OMGOMG" and forget to make things.
I don't know what causes you guys to slip up on macro, but for me it's always a matter of mental discipline. APM is not the limiting factor for me-- it's focus. Practicing build orders and mechanics helps build up that mental discipline to the point where constantly making units always happens no matter how bad things get. After a break from laddering for more than a few days, I really need to consciously remember to do things that used to be automatic-- putting more stress on my decisionmaking and other cognitive functions since I'm "manually" macroing.
In any case, I hope this was somewhat helpful for you guys. I don't know what causes you personally to slip up in macro, but for me it's usually focus. Take some time to think about how you think, and what causes you to make mistakes-- then isolate and kill those cognitive dead ends like you're a mutalisk flock and your mistakes are full medivacs flying past an overlord in unprotected airspace.