From the overwhelming bliss of "what have I done to deserve this" comes the shattering bewilderment of the same line in the reverse context. We are left wondering how we could have fallen into such a shameful mess, behaved so insanely, invited such catastrophic redemption from what we thought would last forever, and once you've got over all that, you're left wondering what it was all for.
To shed some light on the process in addition to the other (incredibly useful) stuff you'll find on Shaza's sites, here's my tick-list based on a TF love I'd unwittingly harboured for 35 years, which needed rekindling in 2014 to enable me to appreciate what I'd been meaning when I'd explained fleetingly to people over the decades that I'd "loved him beyond reason."
Most relationships come and go in a heated rush, unless they're robust enough to end up in a living together/married kind of situation and withstand the long haul. A TF relationship isn't the same. Once 'honeymoon phase' is over, there'll be a trainwreck of sorts, and your Twin will most likely pack up and flee without warning or explanation. The attempt at early union comes to nothing because it can't come to anything based on the kind of physical togetherness you'd associate with a standard tryst. TF is a candle that burns you when you get too close, and draws you to it like a moth. After a 'normal' liaison is over, you'll move on and leave the other party behind in the process. You can never truly leave a TF behind. One way or another, they'll haunt you and there's nothing you can do to stop that from happening.
2) The Obsession
A love affair is usually accompanied by some degree of obsession, which ebbs and dies once you've gone separate ways or cleaved to each other. TF obsession is different. Waxing and waning like phases of the moon, concentration on your Twin will pop up out of nowhere and remind you how you truly feel. In there, you'll get the impression that you're stuck with this for life, and that acceptance is crucial. You don't choose whether or not your Twin is there in your mind, and in accepting this you're letting go. No need to figure out what 'letting go' means for you in the TFD, it's more to do with achieving a balance in yourself that allows you to move through life with someone else sharing the keys to your head.
3) The Healing
This is the point of the TFD. Never mind nesting and procreation, the TF relationship is about learning to heal yourself and helping your twin to do the same. Some writers talk about 'purging the soul', clearing out old baggage, dropping burdens of the past, etc.. Some or all of these descriptions might apply, but however it is for you, there's a lot of healing to be done within the TFD and once you've got past the heartbreak, piteously crying at the persistent thoughts of never seeing them again, you have to get on with this healing process because it's par for the course, it's a primary reason why you were twinned in the first place and there is no point in attempting to ignore that need - you're in it to ascend.
4) The Unbelievable
Ideal matches are forged by matchmakers looking for shared interests, similar goals, compatible lifestyles and other forms of kinship for two people to enjoy. In landing yourself with a TF, you can throw all those out the window. Yes, you'll find shared elements of Self which leave you gasping at how the other could possibly leave it all behind, but you're likely to be treading life paths that couldn't be more different. In the world of physical reality, you're not supposed to be nesting, you're supposed to be flying in cosmic thermals you didn't know you could reach. The only way to get there is by facing tests of loneliness, insecurity and self-worth that you need to get through on your own, in the sure and certain trust that you're going through this together.
5) The Metaphysics
These elements of the TFD came as a complete surprise to me, and that's after 20 years of researching quantum physics. Telepathy is a factor most TFs talk about; some have obtained cast-iron proof with their twins that telepathy is a consequence of the TF relationship. Fairly early in the second union, physical sensations like mild pins and needles began running through my body and making the hair on my arms stand up when I was miles away from my TF and would often hit me when I wasn't thinking about him (yes, there are moments of freedom at every stage of the process! x). I trusted this happened when he thought of me in a certain way, and it also coincides with what has become our 'shadowland' (virtual) communication exchange. That's the non-locality of your entanglement at work. Synchronicity also has a field day with the TFD and you'll find it an irrepressible feature of your life, once you and your Twin are cleaved to your parallel tracks.
Your twinned souls are talking out of earshot from the physical, so we are left in the dark here in the 3D plane while the plans of action 'upstairs' are being laid regardless in 5D. If you worry, you'll invite negativity. So stay in touch with yourself, on your own track, and the rest will take care of itself in its own time.
As my TF once said to me, "You can't change how you feel, but you can change your situation."