Strange it's usually a front line that stretches. Double check your front lines and bridles.

Connect to a tree and stretch the other line to match.

Q-Line (not the pro) works well but you have to buy a spool and make your own line sets.

It's sleeved so you can tie it again if you ever break a line. The stuff lasts years... I've never seen a set that needed to be thrown away because you can just keep repairing it.

Most people put Q-line on the front lines and standard on the rear.