First of all, eeee! Thank you so much!
Second of all, No! Your stuff doesn't suck! I guarantee it just feels like that because you're the one with it in your mind, and you know all the beats and twists, and you know the ending and... you know?
I've been writing fanfic since before I knew what it was; I started posting my stuff online way back when the only place to do that was fanfiction.net and newsboards and, if you had the time to figure out how to do it, Geocities and Angelfire sites.
I had little knowledge back then of how to write properly. My grasp of grammar and tense and spelling were definitely lacking. But I wrote anyway because I had stories to tell, and I was going to share them one damn way or another! And you know what, people didn't much care about the grammar being less than perfect, because they were there for the story... and I was okay at telling a story. And more importantly, if you show love for what you're writing, then it will always be worth reading!
Other than that, the first piece of writing advice I would be inclined to give would probably be to practice writing drabbles. It takes a lot of work to be concise, and restricting yourself to a set word count really helps! Writing something complete in exactly 100-200 words will force you to decide which details need to be left in and which ones are best left out.
You can check out my TMNT drabbles if you want to see what I mean! It's a fun challenge, and in the end coming up with just the right combination of words to get the point across is so satisfying!
My youngest kid, zero_is_a_nerd, is 12 years old, and has posted three drabbles of her own so far to get the method down. She is now working on a longer story that she wants me to beta-read for her. My eldest kid, CharbroilLaFlamme, is 27 and has been writing for years. She hasn't posted anything in a while, but she is definitely still writing!
Another thing I do when I'm writing is keep a list of words that I tend to use too often. I'll search in my drafts for words like "leaned", "stared", "laughed", "looked", etc; and if they appear more than a couple times in a chapter, then I will replace them with other, lesser-used words and phrases, or even remove them entirely. There's only so many times a person can sigh before they start to hyperventilate, y'know?
Anyway, sorry for dragging this out and being fairly incoherent! I was just so surprised by your message! Thank you for making me feel so good!