I'll admit that I'm pretty new to writing fanfiction, but I've been reading it for a long, long time. So take the following opinions with a grain of salt. ;)
I would try to make the changes feel as organic as possible. If you weave them in without making a huge deal of it, people will probably be receptive. But YMMV. You might consider saying something in the summary or authors notes and then just continuing on your merry way from there. I could also envision a scenario where it would be ok to conspicuously address canon-noncompliance within the text, with one of the characters offering explanation or something. See what other writers have done, and go with your gut.
You might be putting too much pressure on yourself to produce something good. Examine the way you're talking to yourself. One small "hack" I use is that I write in Comic Sans for my first draft. The font is so unpolished that it reminds me this isn't a serious final product, and I can just tool around and have fun with it. I also try not to delete anything while I'm first drafting. Even if it feels really questionable, I'll pop some parentheses around it and come back later to see if there's anything I want to salvage.
Styles change with the times. Whatever tropes or cliches that speak to you, do 'em. Be indulgent. Also there are threads here all the time about peoples' favorite and least favorite fanfiction-isms. Epithets (like saying "the blonde" instead of the character's name, or just "he" or whatever) are officially out of fashion. The "there was only one bed" plot device will probably always be popular.
Everybody here is pretty much the same. We all love media, we all love stories, we all struggle with self-doubt, we all want validation. Just be empathetic and tell the stories that make you feel. And good luck! Writing is a joy. :)