I can understand why you need to rant - I've experienced something similar. There's two ways you can go about it - you could be upfront, kindly say you've tried to be ok with it - but its not something I can accept as it's disrespectful and in your eyes emotionally cheating. The other option is to get some distance and wait till he starts paying you more attention, rather than his game. Sometimes showing you care & are hurt, only encourages their actions. I found the latter worked for me in the long run. Hope it helps!
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