grammatical number - It or them: How to refer to a single noun followed by any with a pronoun?

Is the sentence below correct?

I want to say I can't merely rely on self-learning in order to become proficient in my career, and I need better resources.

Although self-learning is an essential factor of success for any career, I believe it isn’t enough to achieve full mastery in them.

Or do I have to say it this way:

Although self-learning is an essential factor of success for any career, I believe it isn’t enough to achieve full mastery in it.

However, as both sound a bit awkward to me, I would appreciate it if anyone could suggest me better substitutes.