In all serious, you can only really get better.
I mean, you realize you make mistakes, it's getting past that that will make you a better writer. If you want to speed up the process, just read a lot and continue roleplaying. I mean, look up some guides, if you want and lay out some strict guidelines for yourself. A good thing to do is maybe analyze your writing and compare it to someone else's? See what you think you're doing wrong and improve on it. It's good to make mistakes, but you can't bog yourself down on 'em. We all make mistakes and we can all have moments where we just write terribly.
I still think I'm a mediocre writer myself, but I want to get better not because I'll gain a job or anything like that out of it, but because writing makes me happy and feeling good about my writing makes me happy.
As well, it's bad to just look at the negative. It's not wrong to look at a piece of work you've done and say, "Wow, I did an amazing fucking job!" as long as you don't let it get to your head. You can also find someone to critique your writing, but don't let their words get to you either. If a friend truly gives you good feedback, they're doing it not because they think you're terrible and should stop writing, but because they want you to get better too.
I had a friend tell me I'm too verbose. I'm trying my hardest to tone down my unnecessary word usage 'cause it fills up the paragraph with needless things and disconnects the reader due to the fact that they have to wade through so many damn modifiers. T_T But, I'm also trying to learn how to write to show and not to tell and that's hard 'cause I made a habit out of writing my characters feelings out and describing them too thoroughly, which is probably why I'm verbose in the first place. When I don't physically tell the audience (my peers) how to feel about a character, it makes them more compelling and interesting as a whole, as well as infinitely more relatable.
Jot down what you think is wrong with your writing. Ask someone to read something you wrote and if they tell you it's perfect and awesome, then tell them to shove it up their ass. If they tell you that this and this is wrong with it and this or this needs more improvement and it compares to your notes, then you've got a solid grasp on what's wrong. In doing that, you can now find out how to alleviate those problems and minimize those mistakes further down the road. It especially helps if said person points out something you missed, etc.
Most people are terrible at evaluating their own work, so it does help if you ask someone you trust to look things over for you. Make sure you tell them that you want actual feedback and not just a, "I love it," or, "That was awesome," cause that doesn't help you and can inflate your ego more, making your writing ultimately stagnant, etc. You want actual critique, no matter how much it hurts you to read other people's evaluation of your work. It'll make you a much stronger person, in general, and a much better writer.
That's my two cents.
EDIT: You also can't compare writing an actual work, like a novel or essay, to Roleplaying. The two former forms of writing take a lot of planning and countless revisions, while Roleplaying is pretty much perfecting your first draft writing (I saw someone say that, I forgot who, but I thought it was insightful). Your post aren't going to be exceptional or anywhere near novel worthy, but that doesn't mean they're bad. I wouldn't stay tied down to one particular post either 'cause people are going to have already read it and editing it a thousand times isn't going to help you much. It's good to look at a post you've made, see your mistakes, edit them the first time if you think you should (I post and then edit my post) and then move on. Learn from the mistakes you made in that post and apply your knowledge to the next one you make and so on and so on.
Again, it could help out if you have someone critique a post, run through it quickly, and tell you what's wrong with it. Mainly cause you aren't going to catch all of your mistakes and you might be extremely biased in one direction or another which can kind of blind you in a way: make you see mistakes that aren't there or aren't as big as you're making them or you might not see your mistakes at all and think it's the best work out there. I'm exaggerating, kind of, but it helps to have a friend willing to give you advice and critique.
