Talk to him in an open, frank, unaccusing manner about what you are feeling. Ask for reassurances. Use 'I' statements instead of 'you' statements. (I feel this way, not You make me feel this way.)
Feeling like a non-part of his life could mean that things are headed in a direction where the two of you part, but it could just mean that some changes need to happen in order for things to continue on in a better way.
In the mean while do some introspection and see if you can figure out what is causing you to feel the way you do. Is it some need being unmet or something else? Do you feel he loves her more than you? Is it something else, or more things, entirely?
Having fears & insecurities is par for the course. We just need to learn how to recognize them and deal with them in a constructive, rather than destructive, way.