Sorry to hear about your negative experience, VCR86. It's another sad occurences that's part of the never-ending arguments about the one-true-way™ to learn a language. Everyone who participate in the spat believes theirs to be right because it worked for themselves or they believe it to work best.
I see the situation similar to aggresive martial art schools tooting their own horns until things get settled in the UFC fighting ring, and sometimes the losing side will respond with "It is the person who lost, not the martial art." But I digress.
We're a welcoming group over here. I'm sure you'll find that many are willing to jump in to help you when you ask for it.
Foundation for Self-confidence in Learning
Find the best way for yourself to learn. You may take some notes from fellow learners, but before you apply the tips, you'll need to make sure you fully understand the context and prerequisite of the various study methods and their philosophy of learning. Each step may have different foundation needs.
As for why native japanese people can learn the way they do, it is because they start from when they begin to listen and memory starts working as a baby. They are in perpetual language immersion when they are born, coupled with a child's extremely high brain plasticity from age 1 to 7, picking up a native language is easy for them. Then, there are also children who picked up and are fluent with more than 5 languages by age of 10.
As we age, our brain plasticity gets more rigid due to deeper specialization and many optimization for adult survival. It takes a longer time to learn something new without any existing foundation to build upon.
So, never compare to others and be hard on yourself. Don't take it too hard from others' dogmatic harsh criticisms that doesn't consider the learner's situation. You only need to compare to your yesterday self, because no one else is like you.
I'll always ask myself "Did I take a small step forward in my effort to improve today?" Be content, no matter how small a step, there is progress. Whether I learned a new letter, a new term, a new language joke, deciphered an anime character's mumblings, etc., all those are progress, and I'm fine with my pace and am in no life-threatening hurry, as my purpose is different from others.
I'll take the opportunity to share my learning perspective since we have similarities of having english as our first language. Hopefully, it'll help you gain some good perspective.
Learning Japanese Using Another Language
In your case, and much like many of ours, we learn japanese using another language (english) but not native japanese to japanese. There's some steps of learning to walk before we go sprinting, so don't throw all those advanced tips away outright.
Let's start with the basics. As an english speaker, we learn the latin-based alphabets. As a native speaker, rememeber how do we learn? Mostly, those foundation steps are rote learning some parts then we lead to inclusion of associative learning.
Look at the first 5 letter sets of english alphabets:
Aa Bb Cc Dd Ee
Looking back now, other than "C", how do we even group the symbols together and recognize the capital letters and small letters (uppercase & lowercase)? How did we learn to read?
I remember being taught "A" and "a" are both "A"s as rote learning. Then I went with "A" for "Apple" as associative learning. This goes on for all 26 sets of english alphabets of both upper and lower cases. You can apply the technique as well for japanese.
Now look at japanese, we have hiragana "あ" and katakana "ア" where both are read "AH" and can be represented as "a" in romanji. So we have 5 sets of A I U E O as あア、いイ、うウ、えエ、おオ。Just like the english alphabet learning.
Fun fact: Hiragana is modern writing introduced in the recent history of the language reform. Previously, there was only katakana (and kanji).
You can think of hiragana as glyphs with curly brush strokes and katakana and glyphs with sharp pointy pencil/pen strokes.
We have something that native japanese doesn't. That is "cheating" or "express-learning" with romaji because we already know how to read and can augment the pronunciation of the letter combinations to make them sound close enough to the japanese version. Japanese school children only learn romaji at 3rd grade, and for them, romaji is not as much of a study aid as it is to us.
However, as english speakers we tend to group alphabets together as we read as though it is english (diphthongs) subconsciously. Often times, I would read romaji of 大切（たいせつ）"taisetsu" as "tai-set-su" instead of "ta-i-se-tsu". That's the pitfall with romaji-only text.
The obvious way here is to have karaoke text of individual japanese kana with romaji below to aid learning, and that would be very helpful to build your foundation to avoid english-reading tendencies and getting prepped up to transition faster to fluency.
Fun fact: English is stress-timed, Japanese is mora-timed.
Mora is akin to a beat in a constant rhythm.
In fact here's an excellent article from the late Dolly-sensei about a japanese learning song "amenbo no uta": The Rhythm of Japanese: Improve your speaking and hearing
Your example of 漫画（まんが）is "ma-n-ga" read 3 mora-timed, rather than other common foreign reading of "mang-ga", "man-ga", etc.
Once you have grasped the mora-time reading, you have leveled up again and ready for the next step!
Up the Proficiency Ladder
Gradually, once you have remembered all the japanese kana, you may find that:
"An all-romaji text by itself adds more confusion than clarity of individual distinct kana in long texts."
And the next step after this when you leveled up further will sound similar but with different subjects:
"An all-kana text by itself adds more confusion than the clarity of having kanji in long texts."
That is the nature of japanese language and how it has evolved into up till recent times. Ambiguity is rampantly built into the language, and each step (of character upgrade) as we learn, helps reduce the ambiguity and increase clarity of intent.
When you are ready, the next challenge to help speed up even more, is to increase immersion level. One way recommended by the late Dolly-sensei is activity-boxed immersion. Start small, pick one small/short activity, everything strict japanese-only. You can watch drama/anime with japanese-only audio and subtitles; Read your favourite light novel in original japanese with a japanese-to-japanese dictionary accompaniment; Japanese-only chat for a short session (Renshuu has many activities in discord).
I'll end my sharing over here. Hopefully, you may gain something on your japanese language-learning journey, in addition to all the other great posts before me, fit for your purpose.
Feel free to ask for help anytime. Be foolish once with a question, and then a little wiser forever more with the answers.
