It is really helpful to hear another person's point of view on this. I think you were right on the point with some of my partially subconscious fears, and pointing out this grieving process is certainly an eye-opener. I do need to think some more about the practical options you suggested, and see what works for me.
I think a big part of the challenge for me is starting to work on those feelings regularly. We have been talking about opening up/polyamory (I know that these are not the same, but it they have been interchangeable in our discussions) for quite a while, but I really put off handling the negative sides of it - dealing with jealousy, grieving and all that - until something is actually about to happen between my partner and someone else. When this becomes an option, I start stressing out, and attempt to deal with all those intense feelings at once in the day or two I have before he goes to meet with that other person. Luckily or unluckily (I can't really tell), none of those times ended up with him actually establishing a relationship or having sex with others, so I keep getting a rain-check on handling my feelings until the next time.
I guess this has to change, since those intense periods of anxiety really bring me down - I am generally a very positive person, and when I read things I write during those times (like my post yesterday, which was written when my partner was with F) I simply don't recognize myself. Maybe if I let some anxiety and jealousy enter my everyday life and handle it a bit at a time, it would reduce the intensity of what I am feelings when our thoughts start translating into reality.