I love my GTI, probably the best car I've owned in 40 years of driving, but as superb as it is, it's still just a car. I've felt like you many times when trading in a beloved motor for a new car, but after a few months (unless the new car turns out to be a dog or a big mistake), then the rose tinted spectacles for the older car gradually diminishes.
Also, with the GTI now being nearly 7 years old, you have to ask yourself the question if you can afford not just tax, insurance, fuel etc on the GTI as a 3rd car, but also any large repair bills which might be due on a car of such an age ? If you are cash rich and can afford it then keep it, why not, but if like most of us, it's more a sentimental attachment than anything else, then sell it.
With the cost of motoring rising (fuel, insurance etc), unless you get pretty good use out of it as a 3rd car, then it's just a pretty paperweight. Just my thoughts of course, YMMV.