300 resi isnt going to help you one bit. You need atleast 700 to even notice that u have better survivability, but from personal experience i feel like i roll best with around 1300 resi.

This late in the season many players are realy overgeared and will beat you ONLY because they have good gear. not much you can do about it with the gear you have atm. You should get atleast full furious/relentless before you can work out a playstyle that works for you. Im not gonna lie, its gona be a pain in the ass the first weeks untill u get better gear. But once u get on like 25k hp and 1100 resi, things are gonna feel much more comfortabel. so...
step one: get gear
step two: dont panic
step three: get gear

as the ratin goes there are two ratings you should think about. (actualy three, but dont think about this)
one is team rating. this is the rating wich "counts". this is the true rating you have earned and can get points for and unlock items for.
second one is matchmaking rating or MMR, this tells whan your calculated "normal" rating is. And it sets you up with the teams wich is closest to your MMR. they may wary in rating dependin on who is playing.

Hope this helps some