If you follow the teaching of Neville Goddard, there's no reason that manifesting a specific relationship with a specific person wouldn't work. I am yet to have my own personal proof, but I have heard enough examples to be convinced.

So, if you sometimes worry about competition and whether this person will meet someone else, and feel like you're in a hurry to manifest them because of this idea...don't be. Obviously, you don't want to be having those thoughts anyway, but...when you apply the technique, it can happen any time. The circumstances don't matter. It has to work, every time. So...relax!

For example, I have had too many late nights trying to force myself to imagine the end result, but can't do it because I'm thinking about time limits and needing to get to sleep. Nights are the only time I get any peace in my house (that I will soon be leaving), and so I have put this pressure on myself. But, it doesn't need to be this way.

We can take it easy, in the knowledge that any time we decide to create a thing, we can do it, if we apply the technique. I hope this is helpful for someone.