Ubuntu boot errors

Hi! I'm a new Ubuntu user (I'm just starting out), and after spending hours and hours configuring my system, I noticed that my PC was taking much longer to shut down (not even 5 seconds at first, compared with over two minutes now). I asked some AIs, and they advised me to use the following command to list the various errors since my last boot: journalctl -rb 0 -p 0..3
Here's the output I got:
mars 25 20:12:17 gybbs gdm-launch-environment][895]: GLib-GObject: g_object_unref: assertion 'G_IS_OBJECT (object)' failed
mars 25 20:12:15 gybbs systemd[1396]: Failed to start Application launched by gnome-session-binary.
mars 25 20:12:15 gybbs systemd[1396]: Failed to start Application launched by gnome-session-binary.
mars 25 20:12:15 gybbs systemd[1396]: Failed to start Application launched by gnome-session-binary.
mars 25 20:12:14 gybbs gdm-password][1379]: gkr-pam: unable to locate daemon control file
mars 25 20:10:52 gybbs pulseaudio[948]: Module module-bluetooth-discover not loaded.
mars 25 20:10:52 gybbs pulseaudio[948]: Module module-bluetooth-policy not loaded.
mars 25 20:10:52 gybbs gnome-session-binary[980]: GLib-GIO-CRITICAL: g_bus_get_sync: assertion 'error == NULL || *error == NULL' failed
mars 25 20:10:52 gybbs gnome-session-binary[980]: GLib-GIO-CRITICAL: g_bus_get_sync: assertion 'error == NULL || *error == NULL' failed

Does anyone know how to fix these problems? And please explain them to me! ^^'

Thanks for your help (and sorry if the topic is in the wrong category)!