First off, it's almost never too late to fix something. It's certainly never too late to learn a better way of doing things.
The canonical reference on what should be a comment and what should not be is probably the comment everywhere privilege description page in the help center. Towards the bottom, that page has two relevant headings: "When should I comment?" and "When shouldn't I comment?". (Emphasis original.) I really do encourage you to look that over, if you haven't already.
There's also the comment field placeholder text providing some brief guidance. On a question:
Use comments to ask for clarification or add more information. Avoid answering questions in comments.
and on an answer:
Use comments to ask for clarification or add more information. Avoid comments like “+1” or “thanks”.
To me, that is the basic litmus test: does the comment primarily serve to make the post better in some way, or does it primarily serve to provide an (alternative) answer?
If the primary intent is to suggest a way in which the post can be improved, and the suggestion to do so will be invalidated by the post owner following that advice, then it's very likely a comment. (This typically, but not always, takes the form of questions, or question-esque phrasings: "Did you consider X?" or "What happens if someone does Y in your world?" or "Tell us a bit more about Z.")
If the primary intent is to provide an answer, which will provide some kind of lasting value and on which there is no clear action for the post owner to take, then it's very likely an answer. This is the case even if the answer is short, or of low quality, or even wrong.
Sometimes answers will be written in a way that naturally involves questions, but they are still legitimately answers to the original question. For example, one could just as well write "What spectral class is your star? If it's a G2V, then you can..." as one could write "If your star is spectral class G2V, then you can...". In such a situation, the choice to pose a question is a stylistic choice that doesn't significantly change the meaning of the text, and therefore doesn't change the text's answer-ness.
Similarly, a comment suggesting that something could be improved can be posted as a statement. "This answer would be better if you elaborated on X." or "It would be easier to answer this question authoritatively if you tell us how Y and Z interact." are both perfectly legitimate comments. Once the post owner edits to elaborate on X, or describe how Y and Z interact, the comment has no lasting value (and thus can be deleted).
In most cases, it's pretty clear which is the case. In some cases, it's really a matter of judgement. My experience here is that some moderators are more lenient, and others are more strict. What we typically don't do, however, is go hunting for comments to delete; but if they are brought to our attention (via flags, or via ordinary browsing), then they can be fair game. Sometimes comments will be migrated to chat, and at other times they will be outright deleted; which button to press is very much a judgement call on the part of the moderator handling the situation.
To me, broadly speaking, comments shouldn't be used to bypass the normal quality control mechanisms of the site. Low-quality content is still low-quality content even when posted as a comment. If you don't have the time to post an answer at that moment, but you have an idea for an answer, consider instead bookmarking the question (either in your browser, or via the star near the voting buttons and the "favorites" tab in your profile) until you have the time to provide an actual answer. You certainly won't earn less reputation that way...
Also, comments are ephemeral. I like to think of comments as post-it notes attached to a post. They are great for quickly jotting down something that you'll need to remember the next day, but they are lousy as archival records.
Comments can be deleted at almost any time for almost any reason (and not just by diamond moderators), and when deleted, will be invisible to, and cannot be recovered by, ordinary users. Questions and answers cannot; any deleted question or answer can be seen by any user with 10,000 or more reputation (2,000 on beta sites), and can easily be undeleted if necessary.