You have good reason to not 'feel right'about our boyfriend. He doesn't share your feelings of love.
What's more, he isn't being forthright with you. If you hadn't stumbled upon his letter to his sister, you would have never found out about this woman named Annie!
You ask me how to bring up the subject. It isn't clear whether you want to talk about his feelings for you, or if you want to force him to come clean about his interest in another woman. If you are honest and tell him that you know about Annie because you read his mail, then there is a good chance that the finger will be directed at you (for snooping). If he finds out that you violated his privacy, he will feel outraged, which will make it easy for him to shift the focus away from him and back onto you, thereby enabling him to wiggle out of admitting what he's been up to with this woman.
you are in a terrible bind here. If you don't admit to having snooped, then you are guilty of sneaking behind his back, just the way he appears to be sneaking behind yours. And, if you don't admit to snooping, then it will be hard for you to use the information that you have found through an 'illegal' search.
You need to step back and figure out what you want to accomplish in this discussion. Do you want to come clean (about your snooping) and insist that he come clean as well (about Annie)? Or do you simply want to him to tell you to your face exactly how he feels about you?
If you want to him to tell you his true feelings, the best way to draw him out is to ask a direct question. For example, 'I have the clear impression that you don't have strong feelings for me, is that correct?'
Once the conversation is under way, you can continue to explore exactly where he sees this relationship going. Does he intend to be faithful to you? Does he want to date other women, etc. , if you so desire.
I have to wonder why you are even going after this conversation. You have already read how he feels about you, so why do you need to have your nose rubbed in poop all over again? It feels to me like you are being a tad masochistic here, which leads me to the larger issue, which is why you would even want to be with a person who doesn't return your affection, doesn't cherish your body, is repelled by your declarations of love, and to top it all off is involved in some fashion with another woman.
You need to look into yourself and figure out why you would even want to stay with him. When you are clearer on your own motives, then you will know what you need to do.
If you decide that this man isn't for you, then you may also decide to skip the true confessions in which he tells you what you already know: he doesn't like you enough for you to waste your time, and he isn't honest enough to invest in.