Spell use on CD confusion

Currently trying to make a frost mage macro work on TBC classic. Nothing complicated in terms of rotation.

Just have it cast Frostbolt by default but if Fire Blast is off CD use that and go back to Frostbolt. I tried cast sequence, I tried putting on two different lines. Not sure what am doing wrong. Swore this was doable.

2 different lines, and spam the button.

/cast Frostbolt
/cast Fire Blast

Doesnt work, only casts Frostbolt since it doesnt have a CD

it will work.

every time you click the hotkey for the macro it casts one line, the next click, the next line, third click back to line 1.

the time you spend spaming it and/or your MS (if your using a programable keyboard/mouse or AHK) is timing it to land on the frostbolt line between every GCD

anyway, try this,
at the top change the dropdown to priority then put fireblast as line 1

/cast Fire Blast
/cast Frostbolt

rather then going in succession (line 1 2 1 2 1 2 1 2 ) it will priority line 1 (1 1 2 1 1 2)
that combined with fireblast having a CD will cause frostbolt to cast when fireblast is on CD