The MMR system feels like it prevents me from getting better

On my main acc I was silver for about 4ish years. But this season I've really grinded the game and Im almost plat, yet through all of this my gains are still low as shit(+17 and -16 to be exact) and they keep lowering as I go through. I feel like I have simply no choice but to abandon the account that Ive played on for 5 years straight because the MMR system is giving me shitty gains.