I sympathise with people who struggle to make friends. It's hard, and loneliness is horrible. I've been in situations where I've struggled to make friends and it was extremely frustrating.
But. I always saw my lack of friends as my problem or the product of circumstances rather than the fault of "cliquey" others who wouldn't include me.
Aren't "cliques" just friendship groups that you don't happen to be part of? And surely it's not their duty to include you if they don't want to? It's up to them who they want to be friends with and it seems odd to get angry at them for not just insiscriminately including everyone.
Definitely, some peope are just Not Nice. But why want to be friends with them anyway? Everyone else is just bumbling along getting by. If they happen to have a group of friends they laugh with at the school gate, good for them.
Don't look to others to validate you. They just don't have the time or inclination to do that.