Vajra:
What you're asking basically leads to the issue of "does reincarnation work as commonly believed?"
I echo what Dave suggested. Tibetan Buddhism is a dogmatic system that has been around for years. Some Tibetan Buddhists do things like 10,000 prostrations a day. I doubt you'll get into doing so.
It may be that reincarnation happens at an I-there/disc level. One larger self sends out many incarnations, and each incarnation is a self in its own right. After meeting his I-there (as described in his third book), Robert Monroe was told that his I-there has existed for "many" years. He stated that he didn't remember. He was told that he wouldn't, because he didn't exist. When it became time for his I-there to incarnate again, the right mixture of ingredients were put together in order to create his incarnation.
Bruce Moen wrote something similar in his third book, even though he tends to express the viewpoint that selfs/probes do incarnate more than once. He was told that he was created as a retriever probe. The right mixture of ingredients were put together to create his current self.
Thomas Sawyer's near death experience states the same thing. He states that only about 5% of Soul incarnates in the physical. The ingredients for an incarnating self can come from a prior lifetime, previous lifetimes, and even other Souls.
When Ron Krueger had a near death experience he met his previous lifetimes which were still alive and abiding in a state of oneness with each other in the spirit World. They didn't incarnate along with his current self.
If I have interpreted the messages I received from spirit guidance correctly, a self "doesn't" reincarnate over and over again. Rather, different smaller selves from a larger self incarnate throughout various time periods in a manner that isn't strictly determined by linear time.
If you consider what I wrote about Monroe, Moen, Sawyer and Krueger above my interpretation makes sense, because how could the evolutionary goal of the common viewpoint of reincarnation be implemented on an individual-smaller self basis, when new selves are created in order to meet the goals of a larger self?
Mendel mentioned Michael Newton's books. Michael Newton wrote that sometimes a Soul will have more than one incarnation take place at the same time. If this is true, what happens when each smaller self returns to its bigger self? Which personality characteristics are maintained? There is liable to be some contradictions. If personality development works in the manner commonly believed, it wouldn't be possible to reconcile the divergent personality traits of two returning selves and the Soul fragment that stayed behind in the World of spirit, because personality traits aren't worked out so easilly (By the way, dave a mbs has found different results than Michael Newton when he does hypnotic regressions. He has stated that Michael Newton provides a lot of suggestions when he hypnotizes people. This can be seen if you read his third book.).
There is another factor to consider. Some people use the term Soul group rather than Disc/Over soul/Higher Self/I-there/Monad. It is possible that they receive information in this way because not everybody is ready to accept the disc/oversoul etc viewpoint. Even when a person writes about the soul group viewpoint, it is understood that group members share their incarnational experiences with each other. Some have written that this sharing is done completely. The fact of this negates the need for a particular self having to incarnate many times. It learns from its disc/soul group members. Plus, there are ways to learn in the spirit World that don't exist here. Supposedly, there are many spirits who never physically incarnate.
Another factor to consider, if people have to get enlightened while in the physical as the Bodhisatva viewpoint suggests, then where are all the takers? I became interested in Eastern teachings about 26 years ago, and I found that there aren't a lot of enlightened people walking around. At this rate we'll all be reincarnating forever. On the other hand, many near death experiencers have found that much of their spiritual selves are recovered during their NDE. This was my experience during a night in heaven experience I had. I instantly went from being a certain that there is no such thing has God, Christ and afterlife, to understanding for certain that these things do exist.
When people do past life regressions, it is possible that the lives of disc/soul group members are tapped into, rather than a smaller self that reincarnates over and over and over and over again. Carried over injuries suggest that this is the case. For example, in his first or second book, Bruce wrote about retrieving a prior self named Joshua. Bruce had a liver disease in the exact same spot Joshua was wounded with a spear. After Joshua was retrieved by Bruce, he had his injury cured. Resultantly, negating the need for Bruce to incarnate with a physical disorder, if reincarnation is thought of in the commonly believed way. Joshua is another part of Bruce in a disc sense, not in a smaller self that reincarnates over and over again sense.
I believe there could be some exceptions. If a self is really unhappy with its past lifetime, it might choose to incarnate again. There also might be occasions where Souls reincarnate for service purposes.
But don't take my word for it.
P.S. You can go nutty trying to figure out this sort of thing. It's really tough while here in the physical. Welcome to the club of the beffudled.