Sounds entirely normal. Love tends to last 1.5 years, give or take a half year, if not otherwise acted on. (Having kids can muck with it.) That's the answer from the last few years of brain chemistry study.
Or, as a polyamorist, i feel compelled to point out that your main complaint is that you no longer are inclined to be monogamous with the girl, so you think this means that the relationship is ending. If that's all that it is, maybe it just means that you're not a naturally monogamous person and you should look into responsible & ethical forms of swinging or polyamory. I don't get the vibe off of your message that this is what's going on, however.
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