Every time I use wp_get_current_user() my plugin breaks

You can get current user ID easily using get_current_user_id().

$usersid = get_current_user_id();
$activity = $_POST['activity'];
$reason = $_POST['reason'];
$explanation = $_POST['explanation'];

For your function f2user() you can change your code to this:

$current_user = wp_get_current_user(); 
return $current_user->ID;

According to wp_get_current_user() doc, if no user is signed in, $current_user->ID would return 0.