Yes, that's to be expected, the same happened to me. The reason being that you probably had mountains of confidence while using the krieg, well so did I and for a good reason - it was OVERPOWERED. After it got nerfed, I became quite trash as well, but after adapting back to the ak again I can tell you - I missed my ak. As of right now, I'm the best I've ever been.
Well to be honest, I was/am just like you. I had huge issues with confidence and when I had no confidence, I would be trash. How did I overcome it?
Well first of all, you need to realize that playing completely passive/default won't get you anywhere.
Secondly - having no confidence = being scared. Why are you scared? Are you scared about losing elo? Are you scared about dying? Well just don't be. Elo is just a number and without taking risks you will keep on losing. Think like this - the enemies aren't much different from you or better than you.
Last but not least - believe in yourself. If you are gonna do a play, don't doubt yourself and always be 100% sure it will work. If it doesn't, it doesn't, whatever. But never doubt yourself. If you are gonna commit to something - be 100% committed.
thank you for coming to my ted talk