One like this isnt very severe. If you make it the right length it will just be there to aid you when you need it. You also wouldn't use it on a day to day basis so your horse wouldnt become dependent on it.
I just perfer to teach my horse stuff without using mechanical devices...in the long run I've seen better results. Also on a side point, I have no money to spend on stuff that I won't be using for a long time. I already need a rain sheet for my horse and a new bit at some point
Your horse is apparently not responding to you. You need to work on your legs and have your horse 'under you' walking trotting and galloping. Now to do that you need to feel the pressure you put in the reins and the pressure in your legs, he needs to engage a good trot with his back legs, you need to 'place him' by playing with your hands, don't pull though!gently open and close your hands still with a good pressure on your legs. It is better to do it naturally than with external help, you first need, before using any artificial help, to do this work by yourself.
Hope that will help!