手伝って くれて 、ありがとう。おかげで早く終わったよ。
I understand it roughly:
Thanks for giving me assistance, thanks to you, the task finished faster
But why it is くれて not もらって?
I thought the context was (I am really thankful for receiving assistance from you and thanks to you the task finished faster)
Also, the person who assists seems to be outsider, so therefore shouldn't もらって be used instead?
And why the くれて is in て form instead of くれた?