Must be something you're not doing correctly, since it sounds personal.
I mean, there should be a lot for you to learn, and maybe even bad habits to correct.
Just looking at the facts here, but you are.. 300 games deep into Bronze 5.
Quality and Quantity both should be equal, where quality is efficient and consistent growth in game knowledge, and quantity is equal to experience / games played.
300 games, 46% win rate, lowest rank possible in the game.. It's just super suspect of 100% quantity, no quality.
I think if anyone is willing to free coach you, it might do a lot more good than harm at this point.
But coaching is a 2 way thing.. You can shove a bunch of kids into a classroom, but it doesn't mean they will all end up understanding the subject completely at the end of everything. Only those who took advantage of the situation given will reap its benefits. You can't just throw down 500$ for a Challenger coach and expect to jump 2 Tiers without seriously taking heed to the information he's feeding you.
Coaching is good if you can't figure out what is wrong with your play.
Coaching is not good for 100% changing the way you play, in the sense that.. even with the necessary information, its up to you to make it second nature that you make the right decision in place of your previous wrongs.
You can know how to make a million dollars within a year. But if you're not mentally strong enough or capable to jump through the hoops to make it happen, then it is not yet reality.
I know your question was about last hitting. But I suspect there's a bigger issue you will run into even after slightly correcting your CS.
It should at least be addressed if you're going to invest so much time into ranked. Unless you don't give a shit, then that's your call.