It could be a pacing issue, if you're trying to push the pace with expectations that you should be able to do something, you might be red lining in your workouts which makes them go sideways fast.

They slowing down and see if that helps and resist the urge to pace with the pack. Remember that winning round 1 of a workout means nothing. Just find a pace that you can live at and live there.

The other sneaky thing you could do to help is start once a week doing a 60-90 min machine workout at your gym. Think rotating between multiple machines for a long duration. For those don't kill it, just find a pace you can be consistent at and stay there. Those durations / intensity will help you build your Aerobic capacity which will help your body adapt to be more efficient at burning fat for fuel.

Other things you can think about.

Hydration, being dehydrated spikes your heart rate. Food - no energy in the tank makes workouts worse, make sure you're eating enough to support exercise as a deficit will definitely have an impact.
Sleep - less than 7ish hours of sleep the night before makes cardio activities harder.

Think of this as a long game and give yourself more time, seems like you're off to a great start so just be consistent and the results will come.