Spring Boot authentication without formLogin() but with custom login form

I'm working on a blog application with Spring Boot Security.
My overridden configure method looks like:

@Override
    protected void configure(HttpSecurity httpSec) throws Exception {
        httpSec
          .authorizeRequests()
            .antMatchers("/users").authenticated()
            .antMatchers("/admin", "/db").hasRole("ADMIN")
            .antMatchers("/**").permitAll()
            //.anyRequest().authenticated()
           .and()
            .formLogin()
              .loginPage("/login").permitAll()
             .and()
              .logout().logoutSuccessUrl("/login?logout").permitAll();
        httpSec.csrf().disable();
        httpSec.headers().frameOptions().disable();
    }

And my custom login form:

<form name="login" th:action="@{/login}" method="post" class="form-signin">
            <h1>Please log in!</h1>
            <div th:if="${param.error}" class="alert alert-danger">Wrong username and/or password.</div>
            <div th:if="${param.logout}" class="alert alert-success">You logged out successfully.</div>
            <label for="username">Username</label>
            <input type="text" name="username" class="form-control" placeholder="username" required="true"/>
            <label for="password">Password</label>
            <input type="password" name="password" class="form-control" placeholder="password" required="true"/>
            <br/>
            <button type="submit" class="btn btn-lg btn-primary btn-block">Log in</button>
            <br/>
            <a href="/registration" style="color: blue !important">You can register here.</a>
            <hr/>
	</form>

The default behaviour of Spring Security is that when I send a request to an URL which needs to be authenticated (for example /users or /admin in my case) it automatically redirects to this custom login page.

I would like to disable this automatic redirection. When authentication is needed I would like to throw a custom exception (which I handle with a separate Class with @ControllerAdvice annotation) instead with message like "You have to log in to see this content". But I would like to reach my custom login page via navigation menu to authenticate "manually".

How could I reach this?

So far I have tried .formLogin().disabled(). In this way I can still reach my custom login page, but when I try to submit it gave an error "Method not allowed". But it is logical since th:action="@{/login}" can't send the username and password to /login.

I have found a solution. Maybe not perfect, but it works.
First of all, I have commented out the login/logout part in my configure method:

@Override
protected void configure(HttpSecurity httpSec) throws Exception {
    httpSec
        .authorizeRequests()
        .antMatchers("/users").authenticated()
        .antMatchers("/admin", "/db").hasRole("ADMIN")
        .antMatchers("/**").permitAll()
        /**
        .anyRequest().authenticated()
      .and()
        .formLogin()
        .loginPage("/login").permitAll()
      .and()
        .logout().logoutSuccessUrl("/login?logout").permitAll();
        */
      .and()
        .csrf().disable()
        .headers().frameOptions().disable();
}

From now when authentication is needed it won't redirect to any (custom) login screen. But the Forbidden (403) error should be handled:

case "Forbidden":
    if (SecurityContextHolder.getContext().getAuthentication().getAuthorities().toString().equals("[ROLE_ANONYMOUS]"))
        error.put("error", "You have to log in to see this content");
    else error.put("error", "It is only for admins");
    break;

Next step is creating a login form into the navigation menu:

<form th:action="@{/loginauth}" method="post">
    <span sec:authorize="!isAuthenticated()">
        <input type="text" name="email" placeholder="e-mail" required="true"/>
        <input type="password" name="password" placeholder="password" required="true"/>
        <button type="submit" class="btn btn-success btn-xs">Log in</button>
    </span>
</form>

After submit we have to be forwarded to a @PostMapping in one of the @Controller classes:

@PostMapping("/loginauth")
public String authenticateLogin(HttpServletRequest request) {
    loginService.authenticateBlogUser(request.getParameter("email"), request.getParameter("password"));
    return "redirect:/";
}

Finally this data should be sent to a service layer to process:

BlogUserRepository blogUserRepository;
@Autowired
public void setBlogUserRepository(BlogUserRepository blogUserRepository) {
    this.blogUserRepository = blogUserRepository;
}
public void authenticateBlogUser(String email, String password) throws UsernameNotFoundException {
    BlogUser user = blogUserRepository.findByEmail(email);
    if (user == null || !user.getPassword().equals(password))
        throw new UsernameNotFoundException("Wrong e-mail and/or password");
    Collection<GrantedAuthority> authorities = new HashSet<>();
    Set<Role> roles = user.getRoles();
    for (Role role : roles)
        authorities.add(new SimpleGrantedAuthority("ROLE_" + role.getAuth()));
    SecurityContextHolder.getContext().setAuthentication(new UsernamePasswordAuthenticationToken(user.getEmail(), user.getPassword(), authorities));
}