I always fail, but this time I want to do my best.
I always get discouraged, but I'm really going to try this time.
挫折する = to fail, to suffer a setback, to be frustrated, to be discouraged
頑張る = to try hard, to do (one's) best, to persevere, to hang on, to give it (one's) all
I always end up giving up, but I want to try hard this time.
以下の表現が考えられます： I always end up giving up, but I want to try hard this time. 「いつも挫折しちゃうけど、今回は頑張りたい」
また、もう一つの言い方も参考にしてください： I usually give up, but I'm determined to work hard this time. 「普段は諦めてしまうけど、今回は頑張る決意をしています」
関連する単語やフレーズ：
end up：最後には〜になる
give up：諦める
try hard：頑張る
determined：決意した
アメリカ合衆国 
