You are going through her private mail. Then you posted it on the internet. I suggest you confess and offer to leave her quietly if she will not forgive you.
You got this all wrong. Its not that she does not deserve you. Its that you don't deserve her.
If she is thinking of leaving you for another, its her right. If you love her, you trust her. You hope she won't leave. But you DON'T meddle in her business or her mail.
You want an answer to your questions? You MAN UP and ask her if she is thinking of leaving you for someone else.
But DON'T ask her if she is cheating, because that is HER business.
If you suspect she is and fear diseases, use condoms.
But as soon as you suspect her and it bothers you, you are pretty much done anyway. In my case, I just stop letting it bother me. Both you and her have rights to privacy, and yes, that includes your sex and dating life.
She is not your property to go through her mail or demand she put her love life on hold for you. And if you think she is, you should just stop dating anyone.