Mernel – Frustration – does the feeling show up in daily life? I guess it must do because your dream features it.
You start out on a London bus, which may mean you liked being in company with others in your life. But that didn’t give you the satisfaction you wanted – a form of frustration? So you made a change – getting of the bus.
The change confronted you – in the dream and maybe in waking life – with an almost instinctive urge – the horse. It depends very much what your associations with horses are, because dream images are made of memories and experiences of things and people. See
http://dreamhawk.com/dream-encyclopedia/association-of-ideas-with-dreams/#Working But the horse carries you, so I see it was an urge or motivation that you were carried along by. But the motivation faces you with so many possibilities or choices that you have to take charge of the search or motivation and lead it down to earth which wasn’t satisfying and led to frustration. Frustration usually occurs when we try things again and again without any success.
It seems you have not reached clarity of what is the way to go in life and you are left alone.
It might help to use
http://dreamhawk.com/dream-encyclopedia/secrets-power-dreaming/ Tony