Hello, Neil.

Encoded into the adventure itself are also all the elements of what you need to get to the mental plane:

Aligned elementals - your lower elementals have to be aligned in order to enable experiencing the mental plane. The distance between the lower two tigers and the upper one might have indicated a gap in alignment between the etheric and astral bodies and the mental body.

You need the aligned elementals to be not upset by what's coming your way (the riders) and to align your state of consciousness with the desired end result, which is an act of balance (as is riding a 20cm bridge on a bike - think Ramana Maharshi's "The Razor's Edge").

Finally, you have to leave behind emotional baggage (your astral backpack). Notice how you weren't even noticing you had it with you. On failed attempts to reach the mental plane it might have similarly subconsciously blocked you.

Balance, alignment, freedom of emotional baggage, intent and perserverance finally led you to your goal. Having done so on your own in a testing environment indeed represents an initiation for you. Congratulations!