I will never paint as badly as I do now.

you need an adverb here (paint badly)

BThis is because I'm making progress, and my arts only gets better with experience.

Only rarely can you start a sentence with because, ie: Because he had learned to swim, he did not drown. (You can also use "since" here)
"Arts" cannot take a possessive, but "art" can.
Instead of "experience", which you could use but is less common in this context, you might say "practice"