Breast feeding preemie babies

My boys were premature, born at 33 weeks exactly. One was 3 pounds 4 ounces and the other was 4 pounds 3 ounces. At first they had to get tube feedings and nutrition through an IV. And then they were able to bottle feed. They are 3 weeks old now and have been eating from the bottle mainly. I pump, so they get breast milk mixed with formula for preemie babies. Feeding is extremely exhausting for them, breast feeding is even harder. So I haven't been forcing it on them. If they get too tired they burn more calories than they take in and lose weight. I have tried to actually breast feed them a few times, but neither of them could latch without the nipple shield. Tonight my big boy, now 5 pounds 1 ounce, latched on right away and breast fed without the sheild for 5 minutes!! I'm so excited and proud of him! I tried with my little boy (without the shield), now 4 pounds 8 ounces, but he still couldn't latch. I will keep trying with him. And now I will breast feed my big boy for a few minutes and follow with a bottle! And slowly increase the amount of time he breast feeds. I'd love to here about other preemie babies and their breast feeding progress/stories, feel free to share!