Not getting invited stings. Throw your own thing. Invite the people who usually get left out.

Being left off a plan hurts in a way that is hard to argue down, partly because it feels like a verdict rather than an oversight. The quickest relief is not proving you deserved the invitation. It is becoming someone who makes plans, since the person who organizes stops depending on being chosen.

Pick a day, pick something cheap, and ask four people, including one who rarely gets asked. Pizza at your house or a park on Sunday is plenty. Keep it small so it actually happens, and skip any mention of the thing you missed.