I tried to comment on this as well. If you wait until you are both completely comfortable, it will never happen. You can't GET comfortable until things happen and you find out it's not as scary as you thought.
So if you are anxious, nervous and a little worried, move forward, I suggest S L O W L Y.
If you are to the point of throwing up or migraines at just the thought (and I've heard people moving from mono to poly or mono/poly have this problem) then you are going too fast! Slow down!
Communicate, not just on things that are happening but how you feel. Not how someone else is MAKING you feel, because that's not fair and it's putting your emotions on others. There's nothing wrong with saying "I feel horribly anxious." Just not "You going out just made me anxious!" When you hit an emotional bump, think about WHY you feel that way. It's a lot of internal work.
DH likened it to emotional yoga. It was hard, made him anxious, and then we stretched, moved forward, and it was a little better each time. Push boundaries and those things that make you uncomfortable until you are uncomfortable. BUT NOT WHEN IT HURTS! That's when you slow down and pull back.
Me: Late 30s pansexual poly.
DH: My husband of 19 yrs and father of 3 teen girls.
DC: LDR of +4 year