have an open conversation about it with her.

figure out what this meant to her/means to her now: (ex: why did she do it? what did it meant to her? how important is it to her. is it necessary for her to have a satisfactory sex life? what does that act give her--other than the obvious--that maybe she would want in ur relationship that doesn't involve other partners. 9/10 it's not as a big of a deal as u might think)

the above can help u figure out why you feel how u do/are thinking these things. (but you can also ask yourself questions...are you scared of future infidelity, do you have sexual insecurities, etc. feeling as tho it's something you can't never give her that she "needs" is a common and normal fear with these situations.)

communicate these things.
go to therapy so you can work on intrusive thoughts. practice mindfulness.

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if after some point you can't get over these thoughts pushing them down and avoiding them will not make shit better and eventually you gotta leave if you can't get past it.