Struggeling with the Forum and HTML

So I wanted to make a thread for something art related, and I wanted to make it look good, of course. Spent multiple hours on setting things up like I wanted them to be and... most of the html disapeared as soon as I hit send.

I just don't know what I've done wrong, a seperate html file I saved looks fine when I open it, the preview looked fine and I only used code I had sucessfully used on my Den page before adding any CSS.
But I'm unable to even center a text in the forum. Why?

I'd gladly share the code here to figure things out, but just don't know a good way of doing so (maybe through dropbox? if yes how?)

Thanks in advance for any and all help, thank you for your time and I hope you have a nice day.
MfG, Zombie

You can use Pastebin to share your code, it doesn't need any registration or smth.

I'll look into that once I get back to my pc.

But is there a reason why not even centering my texts with the tool given in the Forum itself seems to work?

(Test)

Weird, that seems to work, but I hadn't tried it on my phone yet

Hmm? It works as I can see :'d

It should work on phone too

Yes, this time it worked, and I posted it using my phone. Before, any time I tried to center a text in the forum it jumped back to the left when I posted my reply

Ok so this is the code: https://pastebin.com/XUR2tq2w
This is what it looks like while editing:

And this after hitting Edit:

also that reply was centered before sending