Doubt is Good
Consider this situation: I'm a regular person walking down a regular street with a regular brain. I think thoughts, and those thoughts are mine, they belong to me, there is no question of that fact.
I have not developed any ability in my mind to differentiate the source of my thoughts, there is no reason to. If I think, if I hear or feel anything, it is either a sense or my thoughts. Like how a person who has never rode a bike cannot just start biking around with skill, I cannot just begin to differentiate my thoughts with ease.
This is what you do when you begin to force. Like a person who wants to ride a bike, you need to find a way to develop this mental capability, you have to learn to create differentiation between the idea of “this thought is me” and “this thought is not me. Like a person learning to ride a bike, you are going to fall. Like a person learning to ride a bike, you are going to keep falling randomly for years and years to come.
This is doubt. When you have a thought, you have that process go off in your mind, and you just can't quite put your finger on “is that me?”. You can't quite identify, can't quite pick out the needed information to come to that conclusion, so you get stuck. You don't know the answer, in that moment, to if it is really you or your tulpa speaking in that moment.
But here's what you do know.
In the past, before you started forcing, this doubt rarely was a factor in your life. Before you learned about tulpamancy, before you began to force, you never had that doubt. Today, after you have forced, after you have spent time working on making a tulpa, you are finding the question of “is this my tulpa” being difficult to answer, rather than automatic.
That speaks for itself, I think. Doubt is the moment I think people should be able to say “I have a tulpa” with all the confidence of knowing they do. The moment you feel that doubt, the moment you realize you don't quite know sometimes if it was you speaking or not is the moment you know for sure that the forcing and activities you have gotten involved with are changing the way you think. When you feel that doubt, you know that the assumptions you are making about your mind and thoughts have changed.
Do not take doubt as a sign of lack of progress, but as a sign of progress, the first step. The mechanisms that cause you to doubt grow into the mechanisms that allow conversation with one's tulpa, they are not signs that such mechanisms do not exist. Doubt is the signs of the beginning, a foundation you will build on, not a sign of rot or corruption that needs to be gotten rid of or destroyed.