IME, these 4 books are the most commonly cited by CELTA grads:
1. Michael Swan, "Practical English Usage" (grammar reference)
2. Jeremy Harmer, "The Practise of English Language Teaching" (methodology)
3. Jeremy Harmer, "How to Teach English" (methodology)
4. Jim Scrivener, "Learning Teaching" (methodology)CELTA's recommended pre-reading list
includes 1, 2, & 4. They recommend pre-reading 1 grammar + 1 method.
Since CELTA is very intensive, it's probably better to develop as much understanding of grammar beforehand, due to its higher difficulty. Personally, the only grammar (or ELT) book I've ever read or referred to is Swan's "Basic English Usage". BEU is an alphabetized reference, so it's probably less practical than PEU.
Your experience with grammar websites is normal. If any grammar site were very clear & simple, it would get heavy global traffic. Most websites may contain a few gems of crystal clarity, but they'll be buried within a sea of confusion. So, understanding will occur only slowly, gradually, & piecemeal, & with work. Personally, I prefer self-analysis - but that's even harder, slower, & longer.
Try youtube: All english tenses
I haven't yet looked at any of the videos, but I'm sure some of them might be partially useful. I think it's better to first look at the basic whole, before diving in-depth to tense groups, and then finally individual tenses. Check shorter videos with minimalist "boarding" first.
As for passing CELTA, perhaps also try youtubing for CELTA course
or CELTA techniques
. Maybe also look at: the 10 most important things about CELTA
. I've never taken the CELTA course, so I wouldn't know more resources.