I could still feel my breath however the problem was that my breathing became heavier, not as light as before. Now my breath has become lighter, nevertheless after 20 minutes it would become uneven and even heavier. Furthermore, I could only meditate for 30-35 minutes.
You clearly have not received clear and detailed instruction on how to meditate. Your method seems to be aligned with Hinayana meditation while ours originates from the Mahayana tradition. Therefore, I dare not make comment on your approach in order to avoid confusing or discouraging you.
Should you be interested in learning Mahayana meditation techniques, I would recommend that you find instruction from the Chan patriarchs. I personally learned from my late teacher, a Patriarch of Orthodox Buddhism by the name of Xuan Hua whose books I would highly recommend to you. We also have published a book on Chan called “The Chan Handbook: The Learner’s Guide to Meditation” that you can get from this link http://www.chanpureland.org/publications/chanhandbook/.
Furthermore, I encourage you to continue to try and find a competent teacher to guide your meditation practice. As you are discovering, to practice meditation on your own at home is not easy because of the many obstructions that arise and the lack of specific instructions on how to overcome them.
Unfortunately, I do not know of any temple in Ho Chi Minh City to refer you to. I have visited some temples in Hong Kong and Taiwan that are quite competent because they are direct descendants of accomplished Chan Masters.