I don't know how to put it into words so I will repay the gratitude through my deeds.
I am not good at expressing it verbally so I want to say thanks through my action.
言葉より行動がよく気持ちを伝えるーAction speaks louder than words.
行動で恩返ししたと思いますーI want to repay it through my action what you have done for me and I really want to say that it meant a lot to me.
助けてくれて本当に言葉で出せない気持ちですーI can't thank you enough in words. I am so thankful to you.
以上が個人的に使っている表現になります。
どうぞご参考までに。