Spike gets tired of the bad treatment despite the fact that he's helping so he leaves. I really just want to read some fan fiction where Spike leaves Sunnydale.

There are 3 entries in the Search feature for "Spike leaves Buffy":

https://www.spikeluver.com/Forum/index.php?action=vthread&forum=2&topic=308

https://www.spikeluver.com/Forum/index.php?action=vthread&forum=2&topic=979

https://www.spikeluver.com/Forum/index.php?action=vthread&forum=2&topic=979

Although one of them is for a Jake fic and it's my understanding that it's been taken down for rewrites that never happened. However if I saw correctly there were other fics mentioned in the thread.

Peyton
thank you very much

Some other fics that fit what you're asking are:

Come Back To Me by pattyanne
Spike leaves Sunnydale after a fight with Buffy. He's also left her with something else.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=3660&ageconsent=ok&warning=5

Tell Me You Love Me by pattyanne
Spike gets fed up when Buffy won't admit that she loves him. He leaves town. She falls
apart. Help comes from the last person you'd expect.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=2931

Cast Me Not Away by Enigmaticblue
A long series exploring the ramifications of love, loss, and belonging. Spike leaves after "Dead Things," wanting nothing more than to get Buffy out of his head. Wesley's still an independent contractor after the events of "Loyalty." And Buffy's still living in the land of denial.
http://dark-solace.org/elysian/viewstory.php?sid=1012


Not sure if these two fit with what you're asking since they both takes place during season 5. After Spike confesses his love and Buffy shoots him down, he leaves.

Remenissions by adrianiling
Starting during "Crush", Spike asks Buffy why she wouldn't be with him; when she tells him the main reason, Spike makes an important decision and leaves to find what he needs in order to be with her.
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=37140

Fly Me To The Moon by Peta
Set during Buffy the Vampire Slayer Season 5. AU After Crush. Spike decides to return to LA with Drusilla after Buffy rejects him, leaving the Slayer bereft and sending Spike down a spiral of self-discovery that he's not the vampire he once was.
https://www.spikeluver.com/SpuffyRealm/viewstory.php?skin=Blueeyes&sid=5451&index =1