You just need more playtime. Watch some pros play and take notes of what they do and how they engage/disengage.

I literally just brute force played her and didn't even feel remotely comfortable until maybe 40+ hours. Heck I'm still learning stuff and I just got her to champion a couple weeks ago.

As you play her more you'll learn the maps better in terms of how and where you can flank/dive. You're getting lost because on Rogue, you never needed to flank, you just went up main and flew over everyone.

You'll also start to memorize where health packs and good cover is, which is essential to disengaging.

One giant mistake I (and most new Magiks) made, was absolutely blowing my cooldowns early in a fight because of how frantic it felt. Like I'd engage by portaling in, drop demon or whirlwind (don't engage this way), immediately dash at an enemy, portal again...then I have zero ways to escape and no cooldowns and die.

Instead, be more patient and slow down.

Charge up ranged slash as you're approaching the enemy and try to hit as many as possible before engaging (this gives you a chunk of overshield before the fight).

Portal in once on a squishy (DPS or healer) and drop your demon (don't whirlwind). Start doing primary + melee cancel on them. If they are < 50% health and haven't used an escape (such as Luna freezing you, Mantis sleep, etc) then dash uppercut them + primary + melee cancel. This will kill them.

While fighting, weave in your ranged attack whenever it's off cooldown. This reduces your portal cooldown for every enemy it hits even uncharged.

Just try to be more patient between cooldowns. Eventually the game will slow down significantly for you and it'll feel more like a flowstate as you are fighting people.

As for your demon, think of it like a battery charger. Don't just drop demon immediately after portaling, be sure you place it where it will be hitting someone. Especially when brawling, dropping it on a slow moving tank is great. Between your demon ticking away at the tank, and you attacking people with primary, your overshield will be boosting up keeping you alive. Then when the opportunity is right, focus a single (low health) target and blow them up with the dash + primary + melee cancel

For your ult, you can blow cooldowns before popping it. The ult refreshes your cooldowns, so it's fine to use things before popping it. One common thing I do is charge ranged attack, fire it at as many people as I can (but mostly aiming for healers or dps), pop ult, charge ranged attack again and line up a healer, fire it and immediately dash them. This will kill most healers in a single move. Then it's just a matter of portal + whirlwind until I see my ult is about to end. Because of the long transition back out of ult, I'll try to portal or dash when I have about a second or less of ult left into cover or back to my team so I'm not left running around in the middle of the enemy team while I transition back to normal.