Is it necessary to learn Heartfulness from a trainer? Can I not learn it on my own?

The main purpose of learning from a trainer is for them to impart yogic Transmission. As already proposed, you can determine for yourself whether a trainer is necessary or not by conducting a simple experiment. Simply meditate on your own without a trainer, and when you are satisfied that you have fully comprehended that experience, meditate along with a trainer, at which time you will receive yogic Transmission. Compare the two experiences. Only by conducting such an experiment will your heart be convinced. You can reach out to a trainer from the comfort of your home or office by logging into the HeartsApp and requesting an introduction to the practice.