The clauses on either side of the period or comma are independent. Independent clauses generally (in formal written English at least) cannot be joined by a comma without some kind of conjunction.
Your choices are a period (as in the first example), a semicolon, a dash, or a conjunction like "and" or "but" preceded by a comma.
- "Understanding what feeds procastination is only step one. The next step, stopping it, is a bit bigger."
- "Understanding what feeds procastination is only step one; the next step, stopping it, is a bit bigger."
- "Understanding what feeds procastination is only step one — the next step, stopping it, is a bit bigger."
- "Understanding what feeds procastination is only step one, and the next step, stopping it, is a bit bigger."
The differences here are minor changes in tone, although the last one sounds awkward to my ear. To return to the original question, a period would be correct while a comma by itself would not.