You've raised a number of issues. With respect to the company where you worked, you could certainly file an employment complaint and claim that you were wrongfully terminated for sexual harassment that never occurred. This might be more difficult, however, if you were an at-will employee, or if the company has a strict no-dating policy (as I suspect, given that no one else in your company seems to know about your 18-month relationship.)
From what you've said, it sounds like you have a considerably stronger defamation claim. You could probably sue both your former co-worker/girlfriend and the company. A victory there wouldn't get you your job back, but it might help remove some of the stigma you are feeling now and, also, pay some bills until you get back on your feet.
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I obviously cannot advise you about PA employment law, however, the picture you paint , strongy suggests the need for at least a consultation with an employment / labor attorney. Not saying that you are in the clear either as there is the question about the post breakup writeup.
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In addition to the above answers, you should really discuss all the facts and circumstances with an employment lawyer in your city which you can find on Avvo.
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