Difficulty adjusting?

My babies are now 3 weeks old. I don't know if its lack of sleep, feeling bad that we are mostly bottle feeding due to not producing enough breast milk, feeling like I give one twin more attention than the other, or hormones but can someone tell me if this get easier or how long I should expect to feel like I'm not doing this right?!? The boys were born 3 weeks early and are needing to gain weight so we are up every 1-2 hours at night and they eat 2-3oz. Is there a certain weight they reach when they eat more and are sleeping longer through the night? In general any survival tips would be much appreciated for this FTM!!!