My boyfriend (age 23) and I have been dating for a year and a half now, we used to have sex a lot/regularly and then before summer started, it just stopped.
Even though we have tried a lot of new things, he will rarely have sex with me. I'll be lucky if it's once or twice every two weeks. I get so sexually frustrated and I hate it. I hate masturbating (it makes me feel very lonely) and I've come to these conclusions:

A: It's because I've gained some weight over the year.
B: He's gotten bored with me.
Or
C: He's interested in someone else.

I've confronted him about this. He says that when I tell him I want it he gets 'discouraged' and doesn't even want to do it at all. What? How would that make a guy discouraged? Shouldn't that compliment a guy? Or he'll say he's tired, even when he's had three days off work. He doesn't even want me to go down on him, when I'm totally willing to.

I'm afraid he's found someone new, because he's the type of guy who looks for someone months before breaking up with a current girlfriend. He's very charming with everyone and somewhat flirty with his girl co-workers and an ex he is still friends with, but it's completely OUT OF LINE if I talk to a guy friend.

Maybe I'm just over-analyzing everything because he's the only guy I've ever had sex with and I'm his fourth. Please help me out.

You're over-analyzing everything. Stop it with the sex demands. Just be sweet to him and fun and interesting. Don't mention sex at all. Stop being jealous and insecure. You are making ME not want sex either!

I don't completely dog on him though. And I am always sweet to him, I am never mean, ever.

I'm not talking about mean. Do not mention the word sex. Forget it exists. Stop being flirty. Be interesting and fun.

I agree with Wondergirl here. Don't say sex, don't say screw, don't say intercourse, don't suggest it, hint it, draw it, etc. Work on other aspects of your life. Read a book, watch the news, become more interested in what he likes or watches so that you can have a conversation about it. Don't FLIRT! Don't dress sexier. Just be you. Take care of yourself. Love yourself. Be good to yourself. If you can learn to love yourself and better yourself, all the rest of the things you want in life will follow.