gordie, no matter how old or young you are, in your mother's eyes, your happiness and well being is her number one priority, she sounds great and understanding, she understood that before you even left Scotland there may be problems and cancelled the trip, your good health and well being is priceless to her, the £100 is easily replace, don't get worked up over that. You have great parents, they will enjoy themselves in Edinburgh tonight no doubt, it will be like a date night for them, while you're thinking the negative, it may be a good thing for them in the end. Everything happens for a reason, I believe you should try CBT and I would even recommend this
https://moodgym.anu.edu.au/welcome , if you're used to CBT you may find some of this mundane but you can move on and work on some of the exercise given,, it's Australian and well recommended . Like Baldric said, the weather is awful and maybe in next few months you could think about going then