The pain of not being together:
This seems to be the hardest fact to accept initially, you ask yourself, others, God about the reason you felt this deep love in the first place when it was not meant to be. You lose faith in God, you lose faith in fairness, your love gets mixed with bitterness of pain, you start blaming everything around you, but you blame yourself the most. With time you come in terms with acceptance as you get tired of not being able to accept, you go along with life's inevitable flow, and you let things happen, but that pull within you never truly goes away. You realise your love was not here to leave your existence any time soon, you sense the divinity of it even though you may not know anything about spirituality, you realise the purity hidden in the pain of being separate, you feel the pain of being separated from yourself. And this is how you allow the triggers coming from your twin flame to show you your shadows, because if you were done with the love, you'd never truly care about what your person is showing you about yourself, and this is where that “going within" phase starts profoundly.
The pain of facing your shadows:
With every reunion you find out your deeper layers, your darker shadows. Every interaction with your twin flame brings out the expectation, judgement, attachment, abandonment etc, and when you're dealing with this, this seems to be most painful at this time, even though you've already sort of accepted that you might not be able to be with your twin physically. That acceptance doesn't truly wash off the deeper shadows within you, and there are much much inner work to be done other than just coming in terms with this acceptance. Every reunion feels like a new hope, with that comes up deeper layers of needs. However, with every next reunion and separation phase, its noticeable that you slowly understand what really is like to not expect even the smallest of things (eg. Birthday wish) from your twin flame, so you slowly heal the wound which was created due to lack of attention and affection from parents or other elders in childhood. And I just said about expectations, but this way, with every next phase, you notice yourself letting go every need that had once made you feel miserable without it. There are many layers of a single shadow, the more you progress, the deeper you get, the harder it seems, but the more you heal, the more you get close to uniting with yourself. With every phase, that acceptance, that surrendering, becomes stronger, as you shed your earthly layers and start understanding this journey and its motive, with that your belief in union gets stronger, but you lose that earthly picture of union, you believe in the divine definition of union.
The pain of losing things in life:
Now, there's something more than just dealing with your twin flame. After the twin flame encounter, life is never truly the same again, because you're not the same again. The parts that are undesirable within you starts crumpling down, with that the surrounding with which you can not resonate anymore starts falling down. When you notice this happening again and again, in every aspect of your life that you once proudly claimed (friends, family, relationships, marriage, job, studies, position in society, reputation, your belief system) rapidly breaking down into ashes, this again seems to be extremely painful. This is a time when you feel like you're cursed, that everything is going wrong, that it's not about just not being with your beloved anymore, that altogether this life has become pathetic. However, after awakening you start progressing in a different path, and you understand that only things that weren't resonating with you had left you, and that you can now start with things or people that resonate with you. With that it also dawns on you that your twin flame never really left you, you both came across at times, you felt connected to them, you understand this is not something that's supposed to leave you. Eventually you will stumble upon people who had similar experiences as yours, who could feel compassionate towards you, who would not judge you for how you explain your experience. You start hearing your inner guidance towards life goals and decisions without worrying about other's expectations or thoughts. As you connect every incident in your life and realise how it has led you to do what you came here to do, your focus on that earthly definition of union fades more and more, you become more focused on your purpose, your power of surrendering becomes stronger than ever, and you eventually unite with yourself.
Overall this journey is painful, but worth experience, as the pain always leads to a divine peace, it leads to you. Throughout this journey, your twin flame's love keeps you strong, though you're already born very strong. Thanks to the energetic connection between twins, the dreams, telepathy, the feeling of having your twin within you, all signs and synchronicity …every such thing keeps your faith on love, on this connection. Eventually you loose your grip on aspects of yourself that once leaves your being, you become authentic, you get close to God, you believe in divine union. Your twin flame also does show up at times in life and opens up to you about their love for you, and you feel more compassionate to them as you understand their pain as well. And when the thought finally dawns on you….that your twin flame is your spiritual partner, and you both are in this journey together….I think that's when no such obstacle seems like obstacles.
Reference ~ Tapasi On the Twin Flame Journey - Quora