The problem with motivation is that today you have it, tomorrow you don't.
So here are a few tips on how to maintain and improve your motivation:
1. Choose the right method. If the method you are using is good and you are using it the right way, you will improve and that sense of improvement will in turn improve your motivation. It’s what my book is all about.
2. Know your limitations. There will always be frustration, you can’t avoid it 100%, but you can reduce it. Less frustration, higher motivation. There is a whole chapter in my book about limitations on learning a foreign language
3. Set the right goals. A lot of people set very unrealistic and unreachable goals, get discouraged and give up on the language. Setting yourself clear and realistic goals strengthens motivation.