Strikes me that the old adage that a PPL is a licence to learn is never more appropriate here.
15 hours solo is very very little, and I'm sure you appreciate that. Flying with another experienced pilot is a good idea - occasionally. You'll learn from them, and that's great.
But, ultimately, you do need to spend time learning from experience - and that means solo flying. Do be worried about doing it somewhere new - but only worried enough to make you obsessive about getting all of your planning and procedures right. At your current stage of experience, you probably can't overprepare for any flight - and do it as your instructor(s) taught you. Ignore the bar talk about not bothering with (whizz-wheels, PLOGs, met, Pooleys....), just do it by the book, overprepare - and go and learn how to fly the only way it's really possible, by doing it on your own, making some mistakes, correcting them, and learning as you go along.
Also turn all the kit on in the aeroplane and play with it - albeit probably one at a time. Track VORs on a route that you can get right anyway, turn the ADF on and see how it behaves en-route. Learn from it all.
And most of all, learn to enjoy it, otherwise you'll never get anywhere.
G