Practicing is very common and a good mechanism to know how to use if done in a positive manner such as practicing a speech or interview, etc.?

However, assuming relationships will end rather than be successful or feeling you have to practice because you won't be able to deal with unhappiness or unpleasantness in the moment is probably not a good idea. I would work on staying with the relationship and person rather than jumping ahead and missing what you do have now? You cannot be both places; with the other here, now, or in the future which only you can design as it is a fantasy, never exists.