Starfinder general rule: you count as your own ally unless something says otherwise.
Envoy specific rule: If an improvisation allows you to grant an effect to an ally, you cannot grant yourself that effect unless the improvisation states otherwise.
You’re an expert at making people happy and high-spirited, allowing them to press forward when they might otherwise quit. You gain an expertise die of the same value as that granted to an envoy of your level by the expertise class feature. At 1st level, whenever you take an action that restores a creature’s Stamina Points or Hit Points, you can restore additional Stamina Points equal to the result of the roll of your expertise die. You can still use your expertise die on Sense Motive checks, but you always add the minimum possible result. This is also true for any skills you select with skill expertise.
This replaces expertise.
Motvational expertise however, doesn't appear to be an improvisation. So.. that should work on yourself right? You hype yourself up chug chug chug drink the healing potion and bam, 1d6 staminia too.
As long as it's not an improv, I don't see why it wouldn't affect you too.
Might have been intended otherwise, but I don't see the harm in it so, go wild?