The word simultaneous time is the concept that you are looking for in terms of "all at once" You, in essence, experience all of your lives at once. You past life is occurring now, though it is past to you, it is present to them. The future exists as probabilities based on choices, but is happening now. It is future to you, but "now" to them. Each of your "selves" is individual. Together, all your individual "selves" compose your total self, the whole you. Time, a human concept, does not exist. To appreciate the concept, you must expand your awareness to see beyond the concept of time as it exists in human, third-dimensional terms.
Simultaneous time does not eliminate the concept of reincarnation. At best, it only modifies it. Based on the above statements, that information is accessible. You are a part of the whole, though you retain your individuality. You're total self is always present and always acessible (it is often called the higher self, but that's a different topic altogether).
Confusion occurs because we assign human labels to nonhuman concepts. It's not wrong, just distorts the truth. The "ego" is the source of this separation from the total self. It serves as a protective measure. Free will is always present, and your future is shaped on what you experience as a possible (probable) reality. Different choices, different possible realities.
You are correct that the choice to reincarnate is yours. Though initally veiled from you at birth, on agreement, the "past" (pardon the term) is accessible. Upon death, the veil is removed and the seperation you feel is eliminated.
That you are endeavoring to understand this form of consciousness, as it is, you display progression. That you have encountered this now has its own positive outcomes. Those will be for you to discover on your own. My telling you would eliminate your choices and free will.
E.