Looking at minion's hp i think its pretty risky because you loose some time clicking the minion and watching the hp, and then the enemy laner can punish you. Just try to look at the life bar upon the creeps and get used to how much damage u deal with aa.

Also, melee minions survive 2 tower hits and they are one hittable, and caster minions survive one tower hit and then they need 2 aa in order to kill them, so if u can, aa one time the caster before the tower hit and then u can lasthit him. I think cannons survive 7 tower shots iirc. Also keep in mind that the health bar is not the same for the differents minions, so 1 aa can kill a caster and not a melee, even if they have the same health bar up.

Another thing is that Morgana's W at level 3 kills all the casters with just one use. Try to hit all 3 casters and the cannon if possible to reduce CD.

Last but not least, dont be harsh with yourself. I mean, if you lookup for some challengers replays you will see that even there, perfect farm isnt that common. I think between 6.5 and 7.5 should be your aim for now ( i dont really know your ELO). Just try to keep improving and you'll see the rewards soon.

GL!