<human>: Summarize the given text in a few sentences.

Hi Reddit, I've been with my SO for almost three years. 

I caught my boyfriend cheating on me, yet again. This time, I decided that I am not a doormat anymore. A month ago, I went though his phone and found that he was cheating on me. He somehow explained it so that he made it seem like the girl was trying to seduce him, and he was just not interested. I believed it, I forgave him. I just didn't want to fight anymore.

Today he decided to leave his computer on. Big mistake. I'm still having massive trust issues, so yes, I looked through it.

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I found countless Skype conversations with him and an 18 year old girl, dating to over a year ago. Their most recent conversation happened in November (which happens to be when he was cheating on me with the girl on his phone). Unfortunately, we just moved in together in December and neither of us have a way out. 

They have sexted. They have gone on cam together. He has asked her to do things like touch herself for him. They have swapped nudes. He has said he wants to date her. He has said he would stop talking to the 'girls' in his life to have her, because she is exactly his type. The 'girls' being me. It is repulsive. 

I printed off five sheets of their conversations, which included dates from last summer up until November, that pin point him saying very disgusting things. I am going to put it on our bed tonight when I go to class (in two hours). I told him there is a present waiting for him in the bedroom.

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My question is, what do I write as a note to him? The last thing I said to him today is "I don't matter." Perhaps that would be a good one. Is this how I should end it? I am sick of being stepped on. I want to retain the little dignity I have left.
<bot>: My boyfriend has cheated on me for the last time. I want to go out with confidence, and a bang, because I am pissed. What do I do? I am open for any suggestions.