Opening theory is mostly useless unless you are trying to become a titled player. I literally don't know any openings as white against the Sicilian and I still manage to win about half my games. The only thing you need to memorize in the opening are common opening traps so you can avoid them quickly and move on with the game. At your level you will get better results by just practicing tactics, and making sure you aren't blundering pieces. Even if you develop your pieces sub-optimally as long as you aren't giving away free material you will enter the mid game mostly even with your opponent. You should spend the majority of your move time making sure there is nothing hanging or vulnerable. You can miss a game winning move 10 times in a row and still go on to win the game. All it takes to lose a game is missing just one of your opponent's game winning moves. Don't give them that opportunity.
If you can't handle time pressure, play longer time controls. If you still want to play blitz then you have to accept that you will invariably make mistakes and just have to roll with the punches. As for confidence, I don't know its just a game you don't have to take it so seriously. Play and learn for fun.
Uncomfortable playing games after starting studying
Instead of worrying too much about your rating or results in games, focus on understanding the ideas and principles behind the openings you're studying.
@1
"I decided to study some simple openings" ++ Bad idea
"hoping that I would have a bit more control over the opening" ++ Idle hope
"the opposite is happening" ++ That is normal
"I feel unconfident, having bad time management" ++ Remembering < thinking
"blundering pieces" ++ Blunder check before you move
"Does it get better with time?" ++ No, it gets better when you start thinking of your own.
I'm too low to give you chess advice specifically, but in general, I find that learning anything new in any skill-based endeavor makes you worse before it makes you better. You're trying to take some of your auto-pilot bad habits, and replace them with new, better ones. But it'll take a while before those become auto-pilot.
Could that be all it is?
Has anybody faced a similar decline? Does it get better with time?