Glass diamond tile mat backsplash on Formica wall - Tile Advice

[84linze]

Doing a kitchen remodel and all is complete except for the backsplash. The kitchen previously had Formica countertops and Formica backsplash. The countertops have been replaced with quartzite. The Formica backsplash has remained, largely due to the fact that the cupboards stayed in place and trying to remove the backsplash resulted in tearing the drywall paper face off. So it was decided to leave the Formica backsplash in place, give it a coat of bonding primer and use mastic to adhere the tile. Sounded good, but in practice it is proving to be difficult and I’m not happy with the results so far. Here’s the issues: The tiles are diamond shaped glass and stone on a mesh mat. They are heavy and want to shift under their own weight. Being that they are diamonds, standard spacers have only been marginally useful (not 45 or 90 degrees). Any advice?