Hi Rico,
You have already broken the trust between you by invading her privacy and you probably have already jeopardized your relationship. You apparently had trust issues before you invaded her privacy. Otherwise, you would not have felt the need to do so. You said this was your girlfriend, NOT your wife. You also never said how old the two of you are. The reason for the second phone misht be as innocent as because she does a lot of text messageing to her friends and doesn't want you to think she is immature because of it. She might possibly want to keep her main phone line clear so as not to miss any calls from you..... There are a million reasons why she may have a second phone.....even the possibility that she suspects you've been "spying" and becoming too controlling, (which of course you are!) Even if her reasons for having the second phone turn out to be sinister, it is still her right to get it, have it and keep it without having to answer to you for it. It's also her right to be able to trust you to not invade her privacy. You already feel guilty for what you did and you know you were wrong or you wouldn't be asking this blog for help now. I suspect that even if we ALL told you to let it go, it would just keep eating and eating at you until you couldn't help yourself. You would just have to find out what she will say when you confront her. Do you have enough self control to be able to sit down and talk with her calmly and gently? You need to think about this rationally and not convince yourself that she is "GUILTY" of something even though you're not sure just what. It's easy to jump to conclusions about something that might be perfectly innocent. It's also very easy to drive someone you might really care about, out of your life forever because of a jealousy problem. I do hope this works out for you amd your girlfriend. I know it sounds like I'm on the woman's side but actually I'm not. Statistics show that it's usually the other way around. It's usually the woman invading the mans privacy and I would have the same advice for the woman. Whether man or woman, let me say this...."The more you try to control a person.....the further you will drive them away. If you give love, respect, trust and loyalty, you will get all those things in return. If you demand it, without reciprocating, you wont"
Just my opinion.....Good Luck and God Bless!