Hi and welcome from upstate NY. Sorry to say this, but yes, I would rip out what you have done and use a walking foot. You could still quilt diagonal lines. I would go from corner to corner then reverse the direction for the next line of quilting. (Start at the upper left and quilt diagonally to the lower right, then move over however far apart your lines will be and quilt from the lower right to the upper left). Hope that makes sense!
Also, you can quilt your lines farther apart than you want them and go back and fill in more lines if you want them closer together.