I can really relate to your emotional state right now
about two years ago my boyfriend started seeing a new woman and I was very up and down, very ambiguous about everything... but in hindsight it was all ok.
I think you have to remember that it's a process, as trite as it sounds. You really have to move into these things, gradually. I think the state that hurries it along is just being receptive, just appreciating that your mind and heart will take time to adjust.
It's so touching and sweet that you want yourself to feel happy for him, for him to fall in love etc. I think that's a good sign. You shouldn't keep calling yourself petty and silly
You're doing something amazing and new
You feel how you feel, and that stuff goes away the sooner you acknowledge and accept it, and, if your emotions are trying to alert you to something wrong, then you need to address that too, obviously.
If this is something that you largely want to do, and you are vigilant, and you and him love each other, then these are just growing pains. Maybe meet her. Watch how you feel. Be kind to yourself. If it's too scary, back up a little, but don't let your fears get in the way too much...
About the stretching... "a mind that is stretched to a new idea never returns to its original dimension"
Can't remember who said that...