I feel like my best friend stopped caring about me when they got a partner

(On mobile so sorry if it's structured bad)
My (19) best friend(I'll call S)(18) and I have been friends since Freshman year of High school and always hung out. The two of us survived our friend group fighting and people leaving and people hating us, and I always felt like we were ride or die. A couple months ago they got a partner(I'll call E)(19), a close mutual friend of ours, and now I feel like they just don't care about me any more.
Ever since they got their partner they've hung out every single day and I never get to call either of them and just talk to only them, I basically can only talk to both of them. S is also already busy a lot with college work and we didn't game often before (we play a lot of video games together), but now it feels like we never game with our third mutual friend(I'll call J)(19) anymore.
S used to talk about how much their miss their partner while we hung out, but stopped when we asked them to, and constantly rejects gaming offers because of either school work or they want to hang with E, but also wanted us to invite them more because me and J play together a lot now even though they still say "not today" most of the time.
We had a new game we wanted to play, that S suggested, and it's been pushed back for three days now. It's a good reason as their helping E's family with something but it's still frustrating that it always feels like me and J are prioritized last and that S just doesn't care to really try in our friendship. I know S feels bad, sometimes I hear it in their voice, but they never feel bad enough to change anything.
I'm going to talk to them about it soon, and I feel like I'm overreacting a lot, but it really does feel like I don't really mean anything to someone who supposed to be my best friend and that they'd rather spend all their waking moments with E than anyone else. I just don't know what to do when I feel like they don't care about our friendship as much as I do.