"Harden My Heart" is such a good song and the sax & guitar in it compliment each other in ways you wouldn't think they would. I have the same sentiment to harden my heart about love too, sometimes. She is a good sax player and singer...I was surprised by the musical talent in just this song so I went snooping for more songs by them.
"Take Me To Heart" is another song, again with the '80's vibe, that feels like something that would be in some of my favorite Japanimations: A.D. Police Files, Bubblegum Crisis & Riding Bean. They all had a jazzy-rock + hair-metal feel that I think is very Quarterflash.
I really want to look into them more, but these above songs are now in my recent YouTube music rotation.
Here's some more songs by them...