OK, Theerapun, I've thought.
You are right, It's more difficult to change anything when you already have the negative feeling. The thing is to change something BEFORE the feeling comes.
You say that you see images, then you say something to yourself (self-talk) and then get the feeling. Feelings don't come out of the blue, usually there is something preceding the feeling that acts as a trigger. In your case it's most probably your self-talk. (It may be that you are seeing some images before the feeling, but they happen so automatically that you are not consciously aware of it.)
The thing is of course to change something that's happening before the feeling. You see, in situations like this people's brains work like automatically, like they don't have any other choice. It's like robotic behaviour, robots don't have any conscious choice over there doings. BUT PEOPLE DO HAVE! That's the big difference! You have other choices, you don't have to act like a robot. By getting other options for your automatic behaviour you can help yourself.
So. Let's see. You have an image before you tell yourself to go. Let's tackle the image. An here's a WARNING! Do this before you are going to bed! The best time is during the day, when you are alert and don't feel sleepy. And do this somewhere else than your bed.
Sit comfortably and carefully examine what happened last time you were falling asleep. What kind of an image did you see right before you said to yourself "I have to go..."? I'm not asking the content of the image, but location, size, clearness, coloured/not coloured, moving/still... etc. Keep yourself in a good state and don't continue to your selftalk. Concentrate on the image!
And now, here comes the COOL part: Grab the image!! Yes, you read correctly, you can take hold of your images!! (I've done it hundreds of times!) Grab the image and squeeze it into a smaller fist-size object. An throw it away! Do it, do it quickly, that's how your brain will understand that you don't want it.
Now, you probably want something instead of the image you threw away. Imagine the last time you were able to sleep like a baby, without any problems. There must have been such times surely! Just remember one, and see yourself sleeping peacefully and deeply. Now, take this image and move it slowly (in 5-10 seconds) to distance and then move it back, just in front of you, where you had the original image and same size also. And DO THIS QUICKLY! Very quickly! In a fraction of a second! If you are unsure, do it once more, or even a couple of times.
Now test. What happens when you imagine you going to bed, falling asleep? Associate to the situation. What will happen? Isn't it so, that you are unable to continue your old automatic behaviour? Instead you feel naturally relaxed, sure of yourself, and finally sleepy.... snort ....
What I explained to you in the text above is essentially "the swish pattern" (nlp term). The version is of course altered a bit to fit in your situation. If you find it too complicated to do the whole thing the first time, you can do the throwing away part only. It should not cause you any problems. And if problems occur, you can always bring the original image back. (If you have to bring it back, consult your other than conscious mind, is there any hidden purpose for your automatic behaviour.)
I sure do hope that this helps you some! Be well, -Purjo