Mantras are usually given to you by a master, and you are told never to reveal them to others. They are secret. My mantras were given to me by Swami Shivananda. I would never reveal them . I will tell you one thing. Follow the sound of your breath going in and out.
What type of Transition are you seeking in? I am unable to help due to the lack of informations but I am happy to share some key points on Mantra's.
Many indiviudals believe in making your own Mantra is the most effective mantra. As it is personal. Mantras can be created for any reason.
'Compose yourself and breathe deeply. Your breath ushers in a state of peace and calm. With your mind floating and free, focus it onthe idea. Simply sense, without interpreatation, everything that comes to you about the idea; feel, smell, hear, see, acknowledge, taste, and be one with the idea within yourself.
Let your idea be wherever it is. Sense where it is in your body and energy system; let it be. Now invite it to share its own personal sound to you. '
That section is from the Kundalini and Divine Energy, Divine Life by Cyndi Dale.
I do hope this assist you if you need anything else. I will assist to my ability.