i think comfort / familiarity with your partner is a huge part of this + also plain old fashioned communication. if you have an idea for a thread, but you're not comfortable just doing it, reach out to ur partner and ask how they'd feel about it!! another thing is write threads w set goals in mind, so that way you're already both on the same page about what you want to get out of said thread and can easily move yourselves toward said goal.

a small, super easy thing u can do that can help move a thread forward: moving the setting along.

so like let's say your characters start off outside. maybe it's raining. maybe it's hot. maybe ur character just isnt vibin anymore. maybe they're going somewhere!! you can either 1) have your character ask about going somewhere else or 2) simply move it along in your writing, by basically saying they start walking to a new location.

and like, you dont have to do a big dramatic change either. maybe the moving along is as simple as just walking around a park??

one last tip is to offer small hooks, especially for threads that don't have any particular goals in mind. so going with the park thing again. your characters at a park. maybe your character now sees..... an ice cream truck. it'll be up to the other rp'er whether or not they make use of that.

(sometimes rp'ers DO miss these things, so dont be afraid to be kinda glaringly obvious. like have ur character STARE that ice cream truck D O W N. and i think 90% of rp'ers probably won't care if you imply their characters are walking/moving with yours as long as ur not literally going from the north pole to australia.)

addon: i think moving threads along and making things happen is REALLY difficult for all role players, either because they're not sure how to do it or they have anxiety about it!! but in my experience, writing partners love it when you do things to make stuff happen.

A lot of that could stem from, as you said, a personal self confidence issue. My tips are less about 'taking charge' and gathering/presenting yourself in a way that facilitates plot. You don't need to run any big events or make huge waves, start by leading yourself personally and others will follow.

• Pay attention to site dynamics. What factions are in need. What canons are taken. Use the structure of a site or setting, find the holes in plots, and plug yourself in there where possible.
• Read what other people write. It's hard to get a plot if two people / characters have nothing in common. If you throw your character out there with no plan you get stuck in situations you're not enthusiastic about but don't want to bail. Read what other people write and think carefully on what interests them and what you can provide. People love it when you do your homework on them.
• Have a goal for your own character. Pleasant surprises are pleasant but other people can't be expected to provide the font for your character arc(s). Whether it happens quickly over it never happens, having a general idea that can shift over time will help you.
• Put your character/plots in a weaker position. It's easier for other people to accept your plots if your character is beholden to them, or a servant, messenger, idolizes another, etc. rather than potentially forcing them into thinking a certain way about your character. I never say my character is charming, for example, or intellectual, or several other 'good' traits. I play them as close as I think I can get them and accept what judgement comes my way.
• Generate some hooks or stand-out tidbits for your character. I do this for all my characters and sometimes I list them publicly and sometimes I don't and let people piece it together. Example: He always looks down when he walks -- why? He collects baseball cards starting from the year 1997 when his parents took him to his first Nippon Pro game. He's afraid of dogs and will run from them (because he was bit when he was younger). So on and so forth, it makes your character memorable, and someone is bound to want to thread based on a hook or interest.

Apologies if I totally misread your question though.

i agree with everything that's been posted above! i also want to reiterate that communication is key! most people in the rp community are super nice, and as long as you ask and talk to them about what you want to do, they're very willing to work with you, either agreeing to the plot you have in mind or saying no and most of the time, adapting it to work or explaining why it wouldn't.

another thing that i do when i feel like i might overstep boundaries, is asking myself if somebody else were to do come to me with the same thing, how i would feel about it. i think i have a good idea of where my boundaries are "normal" and where they aren't, and in the case where a plot/idea/action in a thread falls under a "normal" boundary, i'll ask myself if said thing oversteps my boundaries. if not, then i usually go for it. if i'm still worried, then that's when i go and ask.

even if you do overstep, i feel like most of the people i've rped with would just let me know and it's honestly not a bit deal. as long as you're (once again) communicating, the two of you will figure out ways to work around it.

thank you everyone for the advice! it means a lot to me! ;w; i'll keep what you have all told me in mind! thank you so much! c: