It sounds like something has changed...and you haven't identified what it is, so you can't really work on it.
Have your feelings for him deepened in some way? (i.e. you were thinking that things were more casual/fun but you found you desires for your future with him changing)
Is it the threat of the new girlfriend moving in that has made you insecure? (how long ago were these plans made?)
Did something ELSE happen in your life that "unbalanced" you? (i.e. did you have a major shift at work, or in another friendship or relationship, that is making you feel more "needy" than you used to be?)
You don't need to answer these questions here - they are just "food for thought" as to why something that previously you were okay with is now bothering you.
I had an instance early on in my relationship with MrS - I thought I was going to be okay with something (him hooking up with his ex-girlfriend/now-FWB while they were at a show together) and it turned out that I wasn't. Turns out that it wasn't that he slept with her...it was that SHE slept with HIM that was bothering me (as she was my FWB too... but our connection was more nebulous) - but it took a while for me to pinpoint where my discomfort stemmed from.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (together 21+ yrs)
Dude: hetero poly male, live-in boyfriend (together 3 yrs) and MrS's best friend
Lotus: poly bi female, "it's complicated" relationships with Dude/JaneQ/MrS; married to TT, poly male
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs on this site:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe