OP you're doing great!! Please remember that, and ss pps have said you've done great to persevere.
My son was 3 weeks prem, had a poor latch and short attention span, breast feeding lasted all of 3 weeks.
I now exclusively pump but he had 2 formula feeds until my supply increased (he's 12 weeks now and it took until he was 10 weeks for that to happen)
I have found that 7 sessions a day for 20 minutes per breast works best for me and I get between 4 and 8oz a time, the latter being my first pump of the day.
I start when he wakes up (between 4.30am and 5.30am) and pump through the day, I try to keep it every 2 and a half to 3 hours.
So I started pumping at 4.45am, my schedule today looks something like: 8am/10.30am/1pm/3.30pm/6pm/9pm. These times will flex slightly as I am out this morning and feeds change too.
Don't be afraid to mix feed if you have to. Is also suggest getting an electric pump if you don't already, I use a manual and it aches after a while.