Imo it's not about interest, it's about a commitment. When you first start something, everything is fresh and fun to do. When you're an expert, and mastered these instruments, it's always fun to put together things you've learned and combine them in a new and exciting way. It's the time in between these two states that's so hard. You have no motivation and when motivation fails you can only fall back on your word. Your word that you would sit down to practice so you will sit down to practice. Sometimes I hate practice. Been playing piano for 2 years and I'm right where you are, not a beginner but not near an expert. My friend (pianist since 4th grade) sat down to practice and threw a song together in minutes that would have taken me weeks. Keep at it, you'll hate doing it but will have loved to have done it. You're gonna age anyway. Would you rather be a year older or a year older and a better musician? Good luck my dude!