I think that's already a lot closer to your real fear. I guess it's something like "I am not good enough". I would look at that instead of trying to get rid of the thoughts of the third party. The root of the problem isn't in the potential third party but in yourself. If you absolutely knew that he eventually will choose you, you would be pretty relaxed about a third party, because you knew it's just temporary for a very short period of time. That's where you want to get. So see it as a chance to look at your own issues and resolve them.
"Self-abandonment. That is the secret. We have to abandon ourselves to the state, in our love for the state, and in so doing live the life of the state and no more our present state. And to make the state alive, one must become it."
Neville Goddard ~ The Law and the Promise