Hi Christina and congratulations on your weight loss! Woo hoo!
Keep in mind that all these formulas are guesses and estimates. Rule #1 in the gym is to listen to your body
and your body seems to be telling you that 142 is too intense for you to sustain. And it's also telling you that you're overtraining. So I suggest that you back off a bit. Personally, I'd call 70% of your max heart rate as moderate intensity, not low intensity.
Instead of focusing on the numbers, how about judging yourself on perceived exertion? On a scale of 1 - 10, how hard are you working? Shoot for somewhere around a 7 - 8 for moderate intensity.
Or try intervals - when you start at say 55% of your max HR to warm up, spend a few minutes at 60%, move into 70 - 75% for a few minutes, then sprint for a minute or two at 80 - 85%, then move back down to 70 - 75%, then 60% for recovery, then back up again. Intervals are an awesome fat burner.
Hope this helps a bit - obviously you've been doing something right so far and you don't want to burn out now.