I remember that sensation, but it was back in brood war for me. It passed when I started studying other players playing the same race as me (allies, opponents from replays or even watch a stream of the race) because I've seen things that simply slipped my mind completely.
Playing the computer in broodwar meant having a strong defensive structure behind which your army would be waiting for the invasion. And yes I would get 10k 10k bank while building the ultimate unit army (battlecruisers, carriers or guardinds+devourers) to kill everything else. I was sure I was the best because I could beat 7 computers on some maps (yes BGH was the easiest of course :p)
But then I started playing against people, that crushed my defensive structures and then my armies. It made no sense and scared the hell out of me. I did not study, I was almost sure they were cheating, did not even want to look at the replay (not that I would have known how to do that - never opened a replay back then, it was a useless button in the interface:)))
I started playing those custom games (turret defense etc.) or 3v3, 4v4 because I could no longer play 1v1.
So I was playing zerg once, in a 3v3, and the enemy jumped on me. I was simply building my wall of 20 sunken colonies... and the allies saved me, and one of them told me privately:
Him: Hey, don't build that defense, build a sunken colony, 6 zerglings and then take another base.
Me: Why would I do that?
Him: Because that's how you play the game.
Me: But I've never played like that and it worked for me. What if they attack?
Him: Well, you defend with what you have and in a 3v3 your allies should help, like we just did.
I remember that discussion because two things happened then:
1. I realized I can use my forces (and not my buildings!) for defense and not only for attack.
2. Employing the strategy he told me the computers completely trashed me, but I started winning games against humans, so that was rewarding in itself :D
That it's pretty obvious to many people, and it was obvious to me after that as well, I guess I just needed someone to spell it out. So... if I were to guess, I would imagine you're getting stuck with the same thing every game and until someone spells it out for you, you'll remain scared.
Maybe share some replays for advice?