Hey how can I say the sentence:

"If you hadn't helped me, I would have made a complete mess of this."

With the same meaning beginning with "But"??

But for the help you gave me, I would have made a complete mess of this.

I don't like that sentence, but it meets your criteria and is correct.

Possibly ...

I would have made a complete mess of this, but you helped me.

But have to stand at the beginning. That is what confused me...

I don't think that's do-able. Another teacher may have a different idea, though.

It's in my textbook... I think it had to be somethink with "But for..."

I thank you! I'm a non nativ speaker and really don't know what to do with that!

Thank you. You're doing very well!