maybe I am going to fast, thinking about seeing with another girl that isn't my J breaks my heart but I can't let this girl down because I am a emotional wreak. but I can't let this girl down because I am a emotional wreak.
I would gently suggest that you could think about reframing that as...
"What kind of partner am I offering to be for this girl when I am an emotional wreck? I need to put my own needs first before I can offer to be a healthy whole person to anybody."
If you still feel that broken over J right now, perhaps your energies are best spent healing self? Don't date.
Or if this is just a first date to help air you out and move away from the previous wacky -- stop putting so much on it. Can't it just be
"I go out on a single date and enjoy it for what it is? A nice time? "
If you mentally load this date up with all the baggage from previous, it points to needing to do more self care first. Try not to project all this anxiety/worry from "the old thing" on to "not even a new thing yet."
YKWIM? Be willing to put YOU first and your healing, health and well being first.