Just Dating Jews
You're really asking two questions here. Though they're obviously related, I think you'll get more clarity if you can look at them separately.
1. How do I get my friend to understand why it's important to me only to date Jews?
You can either explain to her again all the different reasons why this is what you've decided, or you can tell her it's none of her business and stop trying to explain something she clearly isn't trying to understand. If you do want to keep explaining, you can tell her that you really want to have a Jewish home and family, and that you believe that the best chance of accomplishing those goals is with a Jewish partner. You can tell her that your past experiences dating non-Jews convinced you that that's not the path you see for yourself. You can tell her that dating Jewish is a value you're choosing to uphold. If you want to stop having this conversation, you can tell her that enough is enough: This is your decision and you hope she respects your choices just like you respect hers. Then stop engaging in the conversation, and if she brings it up, remind her that you've agreed to disagree, and change the subject.
2. I have a non-Jewish friend who I've considered dating, except that I know I only want to date Jews. What if I'm passing up a great relationship?
If you don't date this friend, you'll never know if the relationship would have been great, or if it would have just heralded the end of a great friendship. You never know what you're missing out on from all the people you don't date, and it's impossible to know. I see a lot of people who are serial daters who can never commit to anyone because there might be someone else out there just that much better. "Love the one you're with," is an enduring expression because it's easy to wonder who else is out there and what could be if the circumstances were different. You could be passing up a great relationship with this friend, it's true. But if you were dating this friend, you might miss out on someone else who might cross your path. There's no perfect system when it comes to FOMO, and you have to decide what makes sense based on what, and who, is in front of you.
3. How does it all fit together?
If you've decided you only want to date Jews, then trust your former self who made that decision, at least for a while, and find some Jews to date. If you compare everyone you meet to this friend, and in a few months start to resent the decision you've made, you could reconsider. You also don't mention whether or not this friend wants to date you, in which case, it might be a non-issue, and the whole question may be a ruse from your Jewish friend to get you to explain your values to her.
The question of interfaith dating comes up again and again in oh so many circumstances. From where I sit, there is no right nor easy answer. You don't want to come across as prejudiced or closed-minded, but you want to be true to your values. You want to fit into mainstream society, and you want to maintain your Jewish identity for you and for your future family. Lots of people figure out ways to do all of these things and lots of couples face these questions both individually and in partnership. You might never get your Jewish friend to understand why this is important to you because it may never be important to her. Enjoy your friendship with her and with your non-Jewish friend, and keep the dating conversations outside of this particular circle.