My advice is, find a way to reconnect with your boyfriend. I know it's super-long distance, but see if you can manage a visit with him, or at least a serious chat via Skype.
If you really want two relationships, it's up to YOU to put in the effort to maintain both of them. It's not fair to your boyfriend that he suddenly became the boring one that you don't feel like talking to because the shiny new boy is more interesting.
After all, your boyfriend was the one who encouraged you to explore things with other people. Doesn't that make him awesome and supportive and open-minded and amazing? You say the new boy is a "deep and sensitive intellectual"--isn't your boyfriend all of those things, just no longer mysterious and new to you?
Weren't you posting on here a few months ago that you and your boyfriend were considering marriage? And the main thing you DID NOT want to do if you opened your relationship was to ruin your relationship with him?
It's certainly possible that dating other people will lead you to realize that your spark and connection with your boyfriend is not as strong as your spark and connection with other people. Maybe you will grow and change and grow apart from your boyfriend--which can happen with any couple. Maybe your boyfriend is not the right person for you to commit to for life.
BUT. If those things are the case, that has nothing to do with your feelings for the new guy--it's about your relationship with your boyfriend.
I suspect you feel guilty that you are less interested in him now, which is making you want to talk to him less, which is making you feel less connected to him, etc. But you do have to talk to him. Communicating with him is what will strengthen your connection to him.
Have you told him about your feelings for the new guy? How does he feel? Was he expecting that you would explore sex a little bit with others, but not fall for a new guy?
If you're now unsure if your boyfriend is right for you, I would keep dating around some more. Don't compare "new boy" vs. "old boy." Explore other options. The new boy isn't going to be exclusive with you, right? You should figure out if you are just dazzled by your feelings for him, or if you genuinely do not connect well with your boyfriend anymore.
Single, straight, female, solo, non-monogamous.