Hi, I'd like to help you with your question.
Anytime you live with someone, there is an adjustment period where you both get to know each other and learn each others boundaries. There is nothing wrong with you trying to adjust by deciding whether or not it is ok to read his mail. Or how much closeness you both should have or how much independence. It is a matter of communication and the two of you working these issues out.
It sounds like you are very fearful of being rejected and feel insecure enough that you don't feel you have the right to make mistakes. So you overcompensate and try to become too attached or prove you are the right person for your partner. That results in lack of respect for you from your partner and yourself.
One of the things you can do is work on your self esteem. Building yourself up and realizing that you are worthwhile and valuable to yourself and in a relationship can make a big difference in how you feel. You also should work on boundaries and communication in relationships. That will help you learn where to draw the line and how to have your own space and give your partner his space.
One of the best ways to help yourself is therapy. Talking to someone who can give you a neutral point of view and help guide you is a key to helping you approach your relationships in a more healthy way. To find a therapist, try talking to your doctor about a referral. Or you can talk with your pastor, if you attend church. You can also search on line at http://therapists.psychologytoday.com/rms/.
You can also work on this issue at home. Here are some resources to help you get started:
Self-Esteem: A Proven Program of Cognitive Techniques for Assessing, Improving, and Maintaining Your Self-Esteem by Matthew McKay and Patrick Fanning
The Courage to Be Yourself: A Woman's Guide to Emotional Strength and Self-Esteem by Sue Patton Thoele
How to Be an Adult in Relationships: The Five Keys to Mindful Loving by David Richo and Kathlyn Hendricks
Boundaries in Dating: Making Dating Work by Henry Cloud and John Townsend
You can find these books on Amazon.com or your local library may have them for you.
Let me know if I can help any further,