Here are a couple things to try:
* First, do a workout in flat terrain, but only breathing through your nose. Start out walking. Over the course of 30-40 minutes very gradually increase the pace. Note at what heart rate it becomes difficult to breathe through your nose. At that HR or just below is your aerobic threshold.
Note that it’s important to approach it gradually to get a good estimation of the threshold. Read this: https://uphillathlete.com/step-by-step-diy-guide-to-determine-your-aerobic-threshold-outdoors/
* Second, on trail runs, stay below that aerobic threshold heart rate, even if you have to walk.
* Third, keep on keeping on! It sounds like you’re on the right track.
I hope that helps.