I’m having issues figuring when to use the different versions of the kunyomi/onyomi for person, fat, and month.
When is it:
I mess it up a lot of times during my reviews, it can also be hard for me to keep track when the jukugo doesn’t use the onyomi reading.
Thanks for the help!
ふた is not one of the readings of 太, so that’s one of your problems solved right there. 
For the other two, I reckon the trick is to learn the readings of words as words, rather than simply two kanji glued together. For example, the pronunciation of 人口 is not some reading of 人 plus some reading of口 - it’s one single word that’s read as じんこう. Once you get the hang of that, you ought to get the hang of intuiting the readings of new words you’ve not seen before.
What Belthazar said; additionally, though, as someone who learns best through patterns, it was useful to figure out that 「がつ」is normally used for specific months (四月、五月, etc), whereas 「げつ」is more for talking about months in general (先月、来月, etc).
As for 人, it’s definitely a lot more erratic but from the relatively low amount of vocab I’ve seen it seems like 「にん」is more common when the term starts with 人 whereas 「じん」is usually the reading the rest of the time. (Again, though, not a hard rule by any means.)
Also obviously I’m the same level as you so I’m not the most experienced, I just thought I might be able to help anyways.
I am with @Belthazar on this, but you may want to read this article which provides some rules of thumb. This problem is the reason we have reading mnemonics!
things got easier for me after taking grammar lessons and using the stuff in sentences or in quizzes.
This is an ongoing nightmare for me. Even when I think I’ve nailed down a word, a little time will pass and I’ll have forgotten it, second-guess myself, try and pick the right reading and grab the wrong one.
I guess I need to use some of these words a bit more practically to cement them in memory.
To add to what @Belthazar said, learn words by their reading primarily, that’s how you’re going to speak them in a conversation. 人口 is not person mouth in my head, it’s じんこう. If I see 人口 in writing, I’m gonna try to match it with what I have in my head.
“Is にんこう a word I know? hmm 
doesn’t ring a bell. What about じんこう? Bingo! I know that, it’s population!”
Relevant:
Whelp that certainly helps regarding 太. Is 太 even a vocab word on its own or just a kanji and then used with other kanji/hiragana. And yes I definitely struggle with seeing the kanji and hearing the reading in my head. I usually am breaking apart the english definition in my head and building the pronunciation from there. I’m trying to emphasize the pronunciation more to myself but it’s hard. I’m hoping it will become more natural as I learn more words and review the same words even more.
@ElleAliaSC
Ooo, that’s helpful regarding the months. Have you found anything that’s helped you target memorizing the pronunciations vs building the pronunciations based on the individual kanji/mnemonics? And yes 人 seems a bit erratic.
@HotWeather
Where did you take grammar lessons? Did you just use a workbook? I got Japanese From Zero but each lesson introduces more vocab and I really want to focus on the grammar and not have to learn vocab through that book and WaniKani.
@febLey
Thank you that was super helpful!
i am taking community classes.
book is: minna no nihongo
Aye, you’ll never find 太 standing alone - nearest you’ll get to it is the adjective 太い.
The way WaniKani does vocab, it would be possible to have 太 as a prefix item. Things like 太腹 and 太物 exist in that prefix usage, but I don’t think it would be a good vocab item to add, because I’ve never seen it.
Sorry, I’m not entirely sure what you mean by this. Could you clarify further?
Mnemonics for remembering the pronunciation of an entire word, rather than just gluing the pronunciation of individual kanji together.
Ah, thanks. In that case, not really anything I can think of off the top of my head. I make up little tricks to remember weird readings for specific words all the time, but I can’t really remember them unless I’m looking at a specific word. (Well, I know I remember 他人 is「たにん」because I kind of treat「じん」as the default reading for 人, so of course 他人 would use its “other” reading, but that’s just an oddly specific example I happen to remember at the moment. Idk.)
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