Everyone exists in all realities to some degree, because we are all one. But we don't exist in all realities with the same degree of intensity.

Say, the person you might miss if you shift your reality, is fully existing with you now. You may be floating, through your creative process, through an area of realities where that person's intensity is very high. Then you will encounter varying versions of that person that correspond very well to the reality shifts that you made, and you succeeded in co-creating with that person the version of that person that you prefer. Yay, you can say! You are creating your own reality, and that person is playing a long really nicely, and you feel satisfied that you understand and enjoy the situation.

You are, however, getting that result in this situation only because that person's probable realities are the most dense within the realities that you are switching to to make your modifications. If you shift into realities where the probability of that person existing becomes lower, that person may still be there, but may feel less close and less important, and if you shift yet further, that person may only exist as memory, or a dream, or an imagination, until she or he fades into the background noise of existence that you are no longer consciously aware of.

So technically speaking it's true that everyone always exists in every reality, so theoretically you can co-create with anyone in any reality. But that won't do you any good when what you prefer is a physical experience of that person, and even a rather vivid experience of that person's creative power, because those only happen within the set of realities where that person has focused most.

In other words- you can co-create any version of any person together with any version of you- as long as the experience is within the probable realities that person is most focused on. That is where that person's free will is to be found- their set of creative probabilities through the realities.

Hence the paradox- the seeming paradox comes from the difference between literal truth (everyone is everywhere at all times) and the practicality of being physical (I have my most meaningful experiences with those who are most focused around me).

So since it is meaningful experience you are after, and a physical creation following your imagination, it is entirely possible to create yourself in such a different frequency that your interaction with one person may come to an end and you may pick up interacting in a somewhat similar way with another person.

Before just accepting that and moving on, however, I would highly recommend to search your belief system for negative beliefs that may dictate the necessity of moving to a reality where that person does not exist. It is entirely possible that, if you change certain beliefs, you will be able to find a reality where that person exists strongly, and where all the advantages that you were looking for in your different reality are also present- or at least those that are most important to you. I would consider finding that a more fine point of living- and perhaps even the metaphysical explanation for a lasting relationship. Mind you, a lasting relationship is not better or worse than a more transient one- and on the cosmic scale, a life-long relationship is just a blip anyway- but if it were the case that for this lifetime a more lasting relationship with that person is what you prefer, then finding and tuning into the band of realities where that person is strongly focused and where all other circumstances also strongly please you is the way to do it.

Have a lot of success fine-tuning your creative power! Do enjoy.

To get rid of possible negative beliefs I recommend Bashar's method: Focus on the feeling that the presence of that person conflicts with your creative desire, and ask yourself: What would I have to believe to feel this? When you get an answer, the belief is gone. If you clear all of the beliefs, you will either find a pleasing reality with that person present, or you will feel loving and okay about drifting away.