As an ESL, one cannot be completely accurate in their ''parallelism'' sometimes. Like this one:
"If I can't tolerate simple criticisms or much worse even, I still couldn't regard myself as a changed person then, I'm still the same person I was before, obviously. Tolerance could be the key, which could help me to reach my goal. And once I am finally at the top, it could be a rather less stressful place as mediocre individuals could already be weeded out.
The main modal verb is could, but sometimes it doesn't look correct(I think) particularly in the very last sentence. Is this correct to you?