When you have a phrase like,
does it feel more like
"I just woke up, so I haven't eaten anything yet" ?
"I just woke up, so I'm not eating anything yet".
The former. For the vast majority of verbs and situations I can think of, it is:
まだ [ te-form verb ] いない
I haven't [ past participle ] yet
I'm not [ present participle ] yet can be expressed with something like:
まだ [ verb stem ] はじめていない
So for example, まだ何も食べはじめていない (literally I haven't started eating yet) is closer to I'm not eating anything yet.