Robbo - you have been given some great advise on here and there isn't much more I can add.
As people have said, it's not a reflection on your ability, but it's exactly as Whirlybird and others have said. All of a sudden, you have replaced your helpful instructor 'passenger' with a passenger who is a distraction and in all likelihood, is probably a little nervous themselves about going up in a light aircraft.
I got my ppl only a few months ago and flew a few solo hours first to consolidate my new found freedom. My first 'real' passenger as in not a pilot friend was my father. I must admit, although being confident in my ability, I was certainly pretty nervous at first to be responsible for the safety of my nearest and dearest. And I am sure that I'm not alone in that respect.
I think you need to treat taking a passenger just like an additional cockpit workload and be aware that there aren't in a familiar environment and explain things. To give you an example, when I did a carb heat check and the rpm dropped a little, it was very noticeable that my father was concerned, even though he didn't say anything!
So being a new ppl myself, I would say just be aware of this and consider that perhaps the additional cockpit workload is something that you initially could do with out?
And congratulations on getting your ppl - well done!!