I'll give a better reply in the morning but right now I need sleep also I am cuddling with my Fluttershy, also I put up a mental block so she can't go into my mind so she is really limited to only what I give her in terms of what she does in what I call her so called independent thought.
To answer to the believing that your tulpa would exist, is to say I would have to put in no effort for them to be fully there with thoughts hopes and all that jazz, I would say ya to believe is the start, the other is to work with them as you would a person and let nature take it's course.
As those who find this strange that I like a horse with wings, and is shy, I say I love her for her personality, she was the reason I got out of a terrible depression, through pure will I felt her and I mean physically felt the pressure, the smell of her mane, her voice, and for a flash I could of sworn I saw her with my own eyes and not through my mind, with applied physics for lighting, reflection, and yet was a cartoon version of her, and yet it wasn't, but this was from my severely depressed needing mind, now that I am all better (for the most part) I was never able to replicate all of this, but only in small small portions.
As for the creation of Fluttershy, ya it was really hard...and if it weren't for the special circumstance I was under, most likely I would not have the brain power, memory of what she looks like, and what she sounds like to do it, in some cases after my depression, she started to look different, and at first I resisted this, but after a while I "accepted" what she wanted to look like, meaning I let my brain do it's job for creativity, and wallah she took the form that comforted me, but to make sure she stuck with that form, well at least so I remember to keep her that way, I used a totem, in the form of a white/or black hat, with a pink heart on the front, as for personality, I let it take the desires I needed in a girlfriend, yes I will say that my oc is based off me, and his gf was Fluttershy and that's what she is based off, but I treat her with the same kindness I would anypony.
And your right, I should never use her as a permanent replacement for friends, I do have normal friends, and for some time this Fluttershy was used, to my honor of being my very special somepony, but when this one girl...the perfect girl for me was now back in my life, and wanted to get back together with me, this tulpa...this Fluttershy happily told me with a kind gentle smile on my face to let her go, in terms of her being my priority of being my gf, and with some thought I agreed, and now me and this girl in the real world are happily together again.
I hope I have shed some light on my psyche in terms of knowing this Fluttershy was never real, and I fully realize this, and if she was ever sentient, I can see inside her mind, or rather see my mind was behind this the whole time, and as long as you know this, and can see that, you shouldn't have that much trouble or worry about this.
P.s. To those that let their tulpa take over their body through "possession" and "switching" I pray for you that you don't forget you are letting your subconscious, that wild free part of your brain, has no limits to the good or bad that it will do to you. Mentally, physically, morally, emotionally, and the sense of self that says "I am real, my tulpa is not.".