Molly, you did the right thing. Knowing your limitations is really important when you take on the responsibility of a young pup.
I have two good friends who have endless experience running two separate Dog Rescues in France. They are always anxious to home a dog, of course, but say it's desperately important to consider where the dog is going, how the 'match' might work, etc., and so they stress every time that they never allow an older couple to take on a puppy. I am repeating what they have always told me, so forgive me if that sounds impertinent since I don't know you or your level of fitness or commitment. However, it can be such hard work when most older people want to offer a home and to have the pleasure and companionship of a dog who is not destructive, not crazy, not demanding, not difficult, not really a worry, or a work in progress.
So, why not think about taking on an older dog, something that will repay you over and over with love and affection for a loving and secure home. It wouldn't be difficult, I should think, to find an older Spaniel if that's what you want.
Don't feel any sadness or regret that you had to hand back your pup. It was the honest and best thing to do and it will settle very quickly with a new owner.
In the meantime, think about an older dog where you not only offer a new life to a deserving older dog who has perhaps lost its owner or suddenly become surplus to requirements, but you then create a vacancy in a rescue centre for another dog to be taken in. Saving two lives instead of one. You also pay far less for an older rescue (or you do in France) and often have Vets bills paid for life and will have loads of time and energy left over for some fun walks with your new dog rather than spending 24 hours a day with your heads on spin, worrying about what a puppy is getting up to all of the time. Don't be sad. Good luck, be positive! ?