The speaker is depressed.
I felt like taking one step forward and two steps back.
The meaning for this part, you seem to have fully understood.
For every sentence I understood, there would be two that I'd get completely mixed up.
Imagine that you have to face 100 questions in an important exam you have to take. First question is really easy: "Phew". You answer it like eating a cake. But the second and the third questions aren't the ones you're going to be able to answer. You skip them. You are able to answer the fourth one, but "fifth and sixth are again 'impossible' to answer." You think.
The trend goes on like this. You answer one question, but are unable to answer the other two. So, out of three questions asked, you can answer one (or at least understand it) but you can't answer the other two.
The speaker isn't talking about a "mold". S\he is talking about a whole language, for instance. If s\he isn't able to understand two out of three sentences, s\he knows this as a setback.
From a more linguistic point of view, the structure after "because" can be used to describe relativity (this isn't the only usage for it; it's mainly used for a bigger purposes, but this is a division of that usage):
For every apple tree in the garden, the gardener planted five new orange saplings.
Generally, you can use this structure (that relies on "for" meaning) for consequences of repeated actions:
For every misfire, the soldier was put for dish-washing duty for three days.