Reddit mentions of English Grammar for Students of Latin: The Study Guide for Those Learning Latin, 3rd edition (O&H Study Guide) (English Grammar Series)

You might want to check out English Grammar for Students of Latin to brush up on your grammar.

English Grammar for Students of Latin

Warriner's English Grammar and Composition, complete course. Stephen King recommends this in his On Writing. This is the one I have been self-teaching from. It has served me well.

Simplest terms? Given that request and your stated language background, I think English Grammar for Students of Latin would help immensely. A lot of the 'not clicking' part might just be missing out on how grammar works even in your native tongue. You can get it for less than twenty bucks on Amazon.

https://www.amazon.com/English-Grammar-Students-Latin-Learning/dp/0934034346/ref=mp_s_a_1_2?ie=UTF8&qid=1549069987&sr=8-2&pi=AC_SX236_SY340_QL65&keywords=English+grammar+for+students+of+Latin&dpPl=1&dpID=41VGnfQiBBL&ref=plSrch

The best book is English Grammar for Students of Latin. I make the students I teach take this book.

https://www.amazon.com/English-Grammar-Students-Latin-Learning/dp/0934034346

It helps because you can easily understand what a gerund or a participle is in English so that a gerund or participle in Latin is easier to understand.

It has little exercises at the end of each chapter to help you practice and apply what you learned about English and Latin grammar.

It isn't very comprehensive, so eventually, you want to use Allen and Greenough or Bennet's Latin grammar. You can download those for free here

http://www.textkit.com/latin_grammar.php

However, as you said, grammar is your bane, so this is why I recommend English Grammar for Students of Latin. It's elementary and will get you up to speed with understanding how grammar works in general so that you can grasp Latin grammar more easily.

I learned the usage before I started school - my mother is a teacher. I didn't learn the grammatical theory of it till high school, and then only because I learned German, which uses the subjunctive mood more than English.

If you're interested (and know a foreign language at least a little), check out the English Grammar for Students of series - e.g., English Grammar for Students of Latin.

If your understanding of grammar needs work, I highly recommend at a minimum picking up a used Latin grammar (like this one or this one) and reading through the grammar explanation bits. That should give you more solid ground, especially with declensions.

edit: Oh, or maybe this.