After React I started getting into the backend and added github OAUTH and sessions on the backend server to save data. They all work fine on the backend and I can access the data from other handlers via sessions etc. But once I try to get the session from the backend using React, I can never do it.

func (h Handler) HandleAuth(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Access-Control-Allow-Origin", "http://127.0.0.1:5173")
    w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
    w.Header().Set("Access-Control-Allow-Methods", "GET")
    url := Oauth2Config.AuthCodeURL("state", oauth2.AccessTypeOffline)
    http.Redirect(w, r, url, http.StatusFound)
}

func (h Handler) HandleAuthCallback(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Access-Control-Allow-Origin", "http://127.0.0.1:5173")
    w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
    w.Header().Set("Access-Control-Allow-Methods", "GET")
    code := r.URL.Query().Get("code")
    token, err := Oauth2Config.Exchange(r.Context(), code)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }

    // Use the access token to get the user's GitHub data
    client := github2.NewTokenClient(r.Context(), token.AccessToken)
    user, _, err := client.Users.Get(r.Context(), "")
    if err != nil {
        fmt.Printf("Error: %v\n", err.Error())
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    session, err := store.Get(r, "session")
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    session.Values["user"] = user.GetLogin()
    session.Values["access_token"] = token.AccessToken
    err = session.Save(r, w)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    fmt.Fprintf(w, "this is authcallback: %s", user.GetLogin())

}

func (h Handler) HandleCurrentUser(w http.ResponseWriter, r *http.Request) {
    w.Header().Set("Access-Control-Allow-Origin", "http://localhost:5173")
    w.Header().Set("Access-Control-Allow-Headers", "Content-Type")
    w.Header().Set("Access-Control-Allow-Methods", "GET")
    session, err := store.Get(r, "session")
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    user, ok := session.Values["user"].(string)
    if !ok {
        http.Error(w, "Invalid user in session", http.StatusInternalServerError)
        return
    }
    // Set the content type header to JSON
    w.Header().Set("Content-Type", "text/plain")

    // Write the JSON data to the response
    w.Write([]byte(user))
}

I tried a lot of things, first I used a different session library than gorilla, it's called scs, I thought it might be my library, but it's not the case. When changing the code I get different errors but the backend works fine every time. On API requests from the backend sometimes I get an empty string data, or a network error or user not found etc, but every time I check the backend in every iteration of the code it works fine. This is the get request:

function App() {
    const [user, setUser] = useState(null);

    useEffect(() => {
        fetch('http://127.0.0.1:3080/user', {
            method: 'GET',
        })
            .then(response => response.text())
            .then(data => {
                setUser(data);
                console.log(data);
            })
            .catch(error => console.error(error));
    }, []);
[]);
    return <>
    <p>Logged in as: {user}</p>
            <button onClick={() => window.location.href = 'http://127.0.0.1:3080/oauth'}>Login</button>

    </>
}