After working out a few years and increasing my aerobic work in the last four or five months, I find myself struggling to get my heart rate to my upper-mid to upper target zone, and I have a hard time keeping it there. If I "slack" off on intensity (reps or resistance) at all, my rate drops like a rock, then I have to struggle to get it back up. Am I trying to work too hard (too high in my zone needlessly?), am I burning out, or what?
One of the adaptations your body makes as your aerobic fitness increases is that your heart rate gets lower at a given workload. Conversely, it takes a greater workload to reach a given heart rate. What you're experiencing is normal and expected.
Whether you are "needlessly" working too hard depends upon your goals. If you're a serious athlete, you need to do some higher intensity training. In most cases, twice a week is sufficient.
If you are doing aerobic exercise mainly for health benefits, don't feel obligated to work in the higher ranges of your target zone. On the other hand, it probably won't hurt you, and it can yield more fat loss and fitness benefits.
If you enjoy working hard once in a while, keep doing it. Just beware of overtraining. Keep track of your morning resting heart rate. If it's ever six to eight beats per minute over your typical reading, have an easy day or take a day off.