I used to be nervous about sharing my writing as well. I focused on connecting with other writers so we could encourage each other. I made a lot of connections and through that, I gained confidence. By connecting with other writers, I was able to get insight and feedback into my own work, but without sharing large portions of it. We would talk in terms of general ideas, plot progressions, or character arcs, and brainstorm how those would fit into whatever we happened to be working on at the time. If you can get with a group of writers like that, you'll learn how to ask for feedback without sharing more than you are comfortable with.
Another option is to work with a writing coach for feedback and assistance. Writing coaches are paid to guide writers in their story creation. Any reputable writing coach won't be in the business of stealing their client's work, or they wouldn't have a client base. It is a professional setting to have your writing reviewed by an expert, without the fear that once it is on the internet, anyone will have access to it.
It is probably also important to determine what you mean by "stealing" in this context. Are you concerned someone will copy and paste your entire project and then try to pass it off as their own? Because that is unlikely to happen, especially if you stick to forums with other writers who are all working on their own projects. Maybe you're concerned that parts of your story could get stolen, like specific characters, your plot, or the world you built? This is where it can get tricky on that path of "ideas can't be copywrited," "original ideas" and "what hasn't been done before."
I've read some pretty intense copywrite lawsuits recently with writers in bitter battles over books that were supposedly "stolen." Addison Cain, an author of Omegaverse M/F fiction, sued another author, Zoey Ellis, who also writes M/F Omegaverse fiction. The lawsuit was because Cain felt like Ellis had based her work off of the books that Cain wrote, thus were plagiarised. The claims she made were in regards to common themes and tropes used in the Omegaverse genre. In the end, Cain's publisher had to admit there was no infringement on tropes, themes, and ideas (because they didn't own them), coming to some kind of settlement with Ellis (this may have resulted in part due to Cain's lying in court and Ellis asking her to be held in contempt and sanctioned for perjury). Either way, this is one of many copywrite lawsuits that show common themes and ideas can't be owned by any one person. Granted, Addison Cain also claims to be the first author to write heterosexual Omegaverse fiction (even though it has existed in M/M and M/F forms on fanfiction sites since as far back as 2010, years before her first book was published, where it originated from a Supernatural fanbase). I digress, apparently abuse of copywrite law is a sore spot for me. My apologies.
Anyway, with how fluidly ideas are exchanged, tweaked, remixed, reinvented, and reworked, you should think about what you would consider "being stolen" in regards to your work. This might help you decide if you want to share some of it online, how you want to share it, or who you want to share it with.