You see, back when you were working with a trainer, the trainer had a training plan. Lunging was part of it probably because of X, Y and Z and your horse fitted in that program to go from A to B and suit your goals. Like SuzieQNutter example : Ingrid Klimke has a plan.
It doesn’t mean this particular program would be valid today in your horse’s condition.
I know you say you aren’t able to afford lessons but maybe one or 2 lessons to be started back into the right track could really be beneficial for both of you.
If not, you are right NOT to use any tools you don’t understand, so my advice still stands for you not to use side reins.
Most importantly, ride your horse to the best of your abilities and have fun. Try not to fight and if you feel something isn’t right, just seek advices from professionals.
Have a great summer bringing back your pony to shape!!!