Why did you start learning English? What makes you keep studying it? I think I've forgotten why I like English. I was not good at memorizing anything when I was a child. I can't memorise kanji nor names of prefectures etc... I always took longer than others. And I ashamed myself for not being able to learn things fast as others do. It sometimes happens now too.
What makes you motivate most is when you use English. Now I'm learning different language and struggling with it. But I feel so happy when I finally can communicate by using it with others. I feel like for this moment, I'm making all my efforts. I think I had the same moment in English, too, long time ago.
Some people asked me what can I do to make students motivated to study English. I used to answer make them speak in English with foreigners. I think it is right. When you can communicate in language you are learning, you'll be so happy and motivated. Espeacially with your friends.
My conlusion is, to motivate yourself, praise & talk with someone in English constantly.