Alright, so this problem takes a bit of background to explain, but in a nutshell:

Having talked with the other players in the group, and having seen how things play out in-session with the party, there is a very real chance that the party will attempt to kill an NPC that is a love interest of my PC ("cause she's evil," and they wouldn't be wrong, pretty sure the NPC is a villain on the edge of being redeemable). If it comes to this, my gut response for my PC would be to either turn on the party or leave the party. I do not wish to retire my character or betray the party, but I also don't want to downplay or ignore the fact the character loves the NPC, as this is currently a strong motivation for the character.

On the front of it, this seems to fall under "My Guy syndrome" (in summary, players sometimes choose to use the explanation "it is what my guy would do" in order to disclaim responsibility for problem behavior, and often it is mistakenly viewed as good roleplay), after all, I don't want the game to cease to be fun for the group as a whole, or to really drag this out. The problem seems to be that while certainly, we shouldn't just remove responsibility by saying its what My Guy would do, there still is a point where if the actions of the group directly and totally clash with a character's motivation, something potentially drastic is going to happen.

What are ways I can try to work with the party, both in and out of character, to either soften or even prevent this potential conflict?

Background:
I am playing in a Pathfinder campaign with a good group of friends. We're on book 2 of Strange Aeons, and one of the elements of this adventure path is that all the PCs have no/limited memory of their past. They just wake up in the same room and in their gut know they should work together. I should also note the theme of the module is Lovecraftian horror, so party paranoia is increased.

Long story short, we had split up because we were running away from a major loss (the party was quite paranoid at the time). On his way back, my character met an NPC that was his girlfriend before he lost his memory. As dramatic narrative would have it, she works for the bad guys (as the group, apparently, did as well before memory loss). Somehow the situation was navigated socially (there were a lot of Casablanca references), and the result of this meeting was that my PC on some level knew he still loved this NPC (and that the same emotions were reciprocated, and the GM even let me roll that Sense Motive check so I'm pretty certain), the NPC provided real and tangible evidence of helping the party (she called off the other cultists hunting the party as a whole, and gave advice where to attack the cult next), she wants out of the cult and is really only there to help a friend, but it was also established that she very much has evil in her alignment ("I've gotten very good at hurting people").

At this point my PC has revealed some amount of information about this NPC to the party (e.g. she exists, is in the cult but wants to leave, she isn't a true believer in the cult, she is helping us). But some things have been left out, such as "she's also good friends with a member of the cult's senior leadership" and "she is the cult's chief kidnapper/executioner/assassin" (I should note here, I'm pretty certain she is an assassin, so in the event there is a party betrayal, this could easily lead to several character deaths, which I definitely don't want to cause). Out of character and among the group it's about the same level of disclosure although I'm more open to saying she really is kind of a villain NPC. As most of that interaction happened out of session, there is a strong sense of party paranoia that I'm either in cahoots with the GM or that there is some sort of other compulsion in effect.

The party is mostly good aligned (LG Inquisitor of Iomedae, NG Occultist, LN Monk, CG Ranger, CG Barbarian, and N Investigator (my character)). The inquisitor mentioned is often quite trigger happy with the Detect Alignment class feature, and there is at least some sense among the party/players that alignment is a valid reason to severely mistrust or even attack an NPC. The barbarian player is also quite often the type to prefer fighting rather than taking a chance trusting an NPC, and has OOC made the argument basically saying "If they're evil and in the cult, then why shouldn't we kill this NPC?". The others are more the kind to be along for the ride.

What are ways for me to navigate this situation and do my part as a player to help maintain a fun game, while still helping maintain the goals and motivations of my character?

I should add, party PVP is fairly uncommon in our group. We've maybe had players have their characters bow out from time to time, but never do we have PCs attacking PCs. When I say "turn" on the party, I mean hand my character sheet to the GM and re-roll on the spot. This is partly why I see this as an issue as well. This line of behavior is not the status quo for our group.

It should be stressed the other party member is an Inquisitor of Iomedae, not a Paladin. They still get Detect Alignment as a class feature. But they at least don't have any sort of code or required behavior. Strange Aeons as an adventure path has some pretty weird concessions with how it handles character backgrounds/classes (such as how anyone could play a paladin/good aligned divine caster). More than likely the party was involved with a cult that was kidnapping people and conducting human sacrifices before they lost their memory. For example, we know for a fact that the Inquisitor had been involved with gathering up certain "troublesome" townsfolk into a house, locking them in, and torching the place. None of our characters have any known affiliation with any given temple or organization. Evidence is actually pointing to the party being former lackeys/cultists of the main villain. Our characters basically just woke up in a cell in an asylum with the knowledge/abilities of whatever class we have.

The GM clearly is trying to play up/into the situation. While I might not have laid out my current thoughts and concerns as directly as I have done here, he certainly knows that I find the likely prospect of the party deciding to kill the NPC troubling for my character. His response was to go over item creation rules for how cheap it would be to make some sort of amulet or item that provides Undetectable Alignment. He also does find the interaction with the NPC to be one of the better established character motivations/roleplaying hooks. As mentioned, a lot of the group is "along for the ride."

The GM has also pretty much said (or at least strongly hinted/joked/coyly didn't comment on) that the NPC has levels in Assassin. Apparently I narrowly avoided having to make some very difficult/important fort saves. In Pathfinder terms this would strongly indicate that the NPC has an evil alignment, so while this is metagaming to assume, it's kind of a foregone conclusion (at the moment I'm banking that the GM is going to give her a necklace of Undetectable Alignment, so that might allay issues with Detect Alignment). Also, her "friend" is likely the person currently in charge of the cult at the moment (who most certainly needs to be stopped), it's not so much that she is helping a friend in trouble, but rather that she is working with a friend of hers who is trying to do some evil stuff.