If your life is so complicated right now and you're struggling with all of this, then why are you still in two relationships with two different people? The easiest way to remove the complication of too many people in your romantic sphere is to break up with your second mate.
I'm a big opponent of the notion that people have control over the actions of their partners. It's not up to you to "allow" him to date other people. You're not his legal guardian. The choice is his and his alone. You can only control your own behaviour and your reaction to the behaviour of others. You can choose whether to remain with him and support him if he starts dating someone else, or you can choose to leave. But you can't choose whether or not he's allowed to date someone else.
It is not about "fairness" like other people are cookies. You have one so... he gets one. It is about what you are willing to be in and can do WELL. If you are not willing, do not play in that shape. Better to be CLEAR about your wants, needs, and limits.
After kindergarten, "fairness" isn't just about how many cookies everyone has. It's also about treating others the way you want them to treat you. I have a huge problem with "I'm allowed to date other people, but you're not." OK, fine, you can't accept non-monogamy in him. Then what on earth makes you entitled to that acceptance from him?
You want him to accept that you fell in love with someone else and wanted to date him, but you're not crazy about the idea of him falling in love with someone. You've made a "concession" that if he falls in love with someone who's already a good friend, then that's OK. But does he even have such a friend? Or are you really just giving him "permission" to do something that isn't even a realistic possibility?
"You get two, I get two"
This is something he told me. And he mentioned I was being hypocritical when I got upset that he might consider adding someone else. I asked him and he said he would be unhappy if he fell for someone and had to choose or reject them.
You
are being hypocritical. What's good for the goose is good for the gander. If you can't take it, don't dish it. You're the one who opened up this can of worms, don't be so startled when they start crawling out all wet and slimy.
GG said, rightly, that you can ask him not to date other people. But you need to realize that this is a request you are making, and he has the right to choose whether or not to grant your request. If he grants your request, he is also well within his right to make the same request of you, with every reasonable expectation that you will grant it.
I don't even know if I have insecurities. I don't think I would have any if his other love was genuine and not just a woman he found. I am just not ready yet. This happened all so suddenly, during a time in my life where I'm having a load of stress at home, a load of things to sort out in my mind. I was not expecting this kind of discussion with him. At all.
I swear I do not know how I would react. I am extremely confused. I don't know if it's the fact that I can't share. I don't know. I only know I'm not comfortable with the idea, I don't know if I will be later or not... I'm just not ready. I wish he would understand that.
It sounds to me like you do have insecurities ("I can't help but feeling that he might find someone who provides what I can't provide, which would lead him to like me less."). Insecurities are about you. They are not caused by him or some hypothetical other woman. They are caused by your self-esteem, how you view yourself and your place in the world, and how much power you give other people to affect your emotions. Sometimes non-monogamy can bring pre-existing insecurities into the spotlight, but non-monogamy itself does not cause those insecurities to exist inside of you.