is SMBB is no ones friend, instant melt can mean many things could be your internet is poor, latency issues, interference of such kind, bad host or connected to shady server or you suck at the game or even the game is BROKEN!!!!.
SBMM plays mind games with you, it will either throw in a bot lobby or sweaty as fuck lobby. From experience after 5 to 6 games you having 2 to 5k/d or higher the game throws me into sweaty as fuck lobby, second scenario the game throws me into a bot lobby but my aim is off; random shit happens and I die. I have sat in a corner before waited for a bot player past and I shot at this guy point black with a shotgun 5 times, guess what? He turned round shot at me twice and I died, watching the kill cam from the shock he emptied a whole slip into me I saw my self just standing still in the kill cam.
All I will suggest is switch to Moderate nat type, see if it makes a difference if the game throws you into a sweaty lobby or with bots and you still not having a good game. Restart your console or close the game app wait 1 or 2 then try again. Now a days I dont try hard anymore unless I'm called out in a lobby then I go ham. When I have 3 to 4 good games, I chill and go easy to avoid been capped by SBMM its crazy but sadly its the truth everyone on here has their experience of this mess.