First off, you seem very defeatist or even depressed about the whole thing. High-functioning autism is a very general term, and many in your situation do have very productive lives including romantic relationships. Sure, you may have to work at it, but you shouldn't automatically rule out the possibility. If you are comfortable going into more detail, that may affect the advice you get significantly.

Now: if you indeed have an imaginary girlfriend, you are obviously not fine with being alone, despite what you say. I'm concerned because if you allow yourself to continue down this path, you may be sabotaging the chance that you do have to find a real girlfriend - as unlikely as that may seem now.

It's really not the "so ronery" part of it that may hurt you later, it's that you may be unconsciously creating unrealistic expectations of relationships and how they should work. If there's a sudden change and you find yourself in a position where you could have a romantic relationship with someone, this may end up damaging that.

You speak as if you're just filling in the holes of reality. The analogy I can most readily think of is this: when you fix a tire with sealant after a blowout, you can keep going on that tire, but you're not supposed to keep driving on it. If you do what you're doing now, you will have very little incentive to find an alternative - someone that exists outside your mind. This is a shame, because you sound like a caring and generous person (from the descriptions of your daydreams, for example).

This may seem difficult (especially in your situation), but I strongly recommend talking to a counselor/therapist/whatever about this in the same way you did here. They won't judge your situation, and they'll give much better advice here than we can in this situation.