You ARE useless. When a new guy comes to class and gets his ass handed to him/her by everyone else there, you'll be the guy that helps to motivate that guy to stick around. You'll also tie that guy up like a pretzel
As for me, I'm insanely small 135lb 5'8"
I think that's like 60-66kg maybe?
Anyway, everyone is bigger than me in my class. What I do is try to get myself into neutral positions and stall them out until they make a move(creating some space for me, etc) and I explode. Keep in mind that you won't beat anyone in class this way, but you will get much better and when it comes time to compete in a tournament with people your size you will be thankful you roll with huge guys.
My first tournament with people my size was amazing. I got caught by technique and still lost, but I was able to take guys my own size and pretty much throw them around. So yea, my lesson from that was to stall and wait for a move on their part, then capitalize.
They're going to try and muscle out of my guard? Spend 5 minutes just squeezing your legs as tight as can be. Make them REALLY work to pass your guard. Make them REALLY work for a mount. Small victories at first, but over time you will start to catch the guys who have really worked you over in the past. And most importantly, you will have earned it.
Also if you are supposed to be working on technique, you could ask them to dial it down just a bit until you get the technique and then increase the pace. Maybe ask to roll with your teacher sometime. Always let your partner know in advance what you are looking for. Some will ablidge, others won't and you can roll with someone else. Just keep working and keep pushing.
BJJ is not something you become good at in the first 10 years. You really start to learn once you get your black belt. Think of it like
White-Blue = Introduction to positional awareness
Blue-Purple = Learning moves/sweeps/guards/passes
Purple-Brown = Making progress toward your diploma
Brown-Black = Sending out college applications
Black- > = First day of school all over again.