Hello Summer and David,
Welcome to our forum.
Sorry if we came across as a little critical; we mean you no harm, we just don't want anyone to get hurt.
If you go to the bottom of this page (on the right side), you'll see a "Forum Jump" menu. You can click on that menu and see a "Dating & Friendships" area. That might give you a place to start looking for what you are seeking.
Hopefully you'll find what you're looking for, but do bear in mind you're looking for something very specific, so it may take awhile to find that special someone (even though you're not necessarily asking her for a lengthy relationship; I understand that).
As for your fear that he will eventually develop deep feelings for the other woman that cause him to leave, you don't have to "just get over it," but you should talk about it with David. Maybe he's just not giving you the verbal reassurance you need. You mentioned that he has a hard time expressing his feelings.
Polyamory can be a complex lovestyle, and sometimes things turn out differently than how we envisioned them. That doesn't mean things have turned out badly, just that they've turned out (somewhat) differently.
Don't know if this helps a little; if you have some more questions just let me know.
Love means never having to say, "Put down that meat cleaver!"