There are some good answers here already, and OP is probably right about simply choking in his promo's and that's all she wrote most likely, but there's one thing I don't see people talk about because I suspect people just don't realize this about this skill bracket, but it particularly plagues low diamond. If your skill plateaus at exactly low Diamond, it could take a massively long grind to actually get out of Plat. The reason is, low Diamond/high Plat is a big smurfing ground. High Diamonds/Masters/Challengers get bored and smurf and/or boost their friends/strangers, and what happens is your odds of getting a truly even game are pretty small in this elo IF you are not smurfing/climbing significantly already. Your odds of getting one of these smurfs is also not in your favor if you are in fact plateaued at low diamond(Not saying you are but it sounds like it since you're struggling so hard in promos) since getting a smurf/booster on the enemy team is greater than one on yours. One can argue that getting a bad player is also greater on the enemy team than yours, but this does not balance things out because you are plateau'ed. If you were, say, deserving of a much higher elo this would be more meaningful but for someone who is trying to just inch into their peak Elo and struggling, getting a smurf on the enemy team is far more crippling than getting an elo inflated player on the enemy team is.

It's also important to note that early on the in season, the ladder is a bit different than in the last few months of the season. We're closer to the middle now but yeah, you may find it significantly easier if you wait. Good luck in the future.