"Now I'm really enjoying myself, I'm telling him don't stop, keep going and so on. Wouldn't you know it, he stops and decides to change positions."
I hear you and understand when you say this and have cried myself and I've never faked it especially with my very longterm partner. I've gotten so frustrated I've just got up and left especially since he always gets his fun first.
Discussion I found and clearly explaining your needs outside the bedroom is warranted in this case and the only thing that has worked for us. It sounds like you're communicating to him during your time what your needs are but a clearer definition and explanation of the cause and effect of his actions might help him to understand your frustration.
Try this time old trusted formula, it works in all aspects of my life, business, personal and casual:
When you do ________ I feel ________ .
This will open the discussion where you can both offer alternatives and other suggestions and ideas. Again, I would keep the discussion outside the bedroom and prior to intimacy. Our bedroom is our safe haven away from it all, a place we can securely nest and relax with one another.
Hope this helps.....