Great question Kelly, and you are not alone in making this transition. 90 is on the high end of the cadence we recommend (85-90), and it can be challenging for many runners to maintain, especially at first. My impression is that you are working harder at 90 rather than relaxing into 90; chances are you aren’t activating a bit of gravity by gently falling from your ankles - if you are upright or bending at the waist, 90 will feel harder. Having someone video tape you from the side while you’re running is a good way to see if you’re leaning slightly from the ankles, relaxing your legs, landing with your feet under your hips, and not pushing off with your toes.
You can also try increasing your cadence very gradually. I’d suggest just increasing the beat by one (81) on your next run. If that feels good and you can complete your workout, go up to 82 the next time. Continue increasing from there. Since you’re trying to build your stamina, make sure you can do the runs comfortably. You want to build your aerobic base gradually so that your body creates more capillaries to carry more oxygen to your muscles. This will make increasing your distance over time much, much easier. I hope these tips help!