A good half pad is good. Especially the mattes ones which you can put shims in depending on what your horse needs. I’d recfomend getting a saddle fitter out. In the mean time I’d say a good breastplate that goes around the chest of your horse and connects to your saddle. Normally it has rings attached but if you don’t use a martingale you can just remove them!
Your saddle doesn’t fit correctly. If you can’t afford to have it fit, you could try a half pad that raises the back of your saddle so it’s a bit higher than the front (so will even put when your weight is added). A tack store should be able to point you in the right direction pad wise.