i used to main demo (still play him alot)
first of all use the iron bomber, its easier to use. if you still can't aim, use the loch n load. deals more damage and has faster projectile speed, perfect for aiming practice. don't use the loose cannon if you can't aim, it takes alot of practice.
it's also important to remember that your grenade launcher will deal 100 damage every shot (enable damage numbers). it will 2 shot light classes and soldier and 3 shot a heavy. if they're overhealed (look for particles), add an extra shot (two for overhealed heavies). this is important
you could also practice using a different class entirely; any class that can use a projectile is good (esp the huntsman, its stupid easy to use). most of the time the arc doesn't matter because it's quite an accurate weapon but you can use the arc if you want to chuck grenades at a sentry from behind a wall (idk what map to use as an example but theres a couple times where this comes in handy)
also try equipping a shield (chargin targe works best for this, tide turner if you a pusillanimous individual) instead of a sticky launcher so you live long enough to connect the shots if you're bad at dodging