So, a couple things I would say,
As for the continuing the improve part I would recommend this, keep acquiring game knowledge, ask yourself before every game, what is the enemy teams win condition? How do I throw a wrench in their win condition? What is our teams optimal defensive rotation? What is our win condition? How do I achieve this? What is the enemy teams cooldowns? What do I need to look out for?
First lets start with the basics, have a good defensive rotation with your healer setup, and dont be afarid to ask question/communicate with your healer, hey what do you want me to do when this hunter pops his cds or hey im going to port the opener if a R/M opens on me. You generally want to avoid defensive overlap. Defensive overlaps, unless you absolutely need to is a big nono.
Have an idea of what your ‘goes’ look like. Dont just dump your offensive cds on cooldown. For example, if you’re running with a hpal, you seem them hoj? Probably a good time to start your go, stun dps and fear off that hoj then ideally your hpal could rep off your fear. Or your hpal could run fast hoj and you guys could just do a hoj/fear go every like 30 seconds.
Id say as a warlock your best healer is probably rdruid, and whenever you cant open a gap, your rdruid needs to just spam clone/root so you can just limp away. Granted im not sure how mobile demo lock is compared to afflic right now.
Lastly, try to record some games then post them here, WoW arenas can be fairly intricate, and its much easier to help if we could see your games and then critique them.
Lastly, as a caster you need to get good a juking kicks. Try to notice patterns of the other team? Do they kick quickly? Or do they wait? Do they mix up their kick pattern, if so how? Generally a good starting point for learning how to juke kicks is this pattern, quick fake, medium fake, then long last second fake. Then try to adapt.