My gut feeling based on your posts thus far, is also that there is a lot of mid life crisis type stuff mixed into all of this--and that in spite of your feelings to the contrary, sou ARE being unrealistic about how hard it will be to be on your own.
You need a plan to be bale to not need other people for support (other than a normal amount of child support) within, say, one year--and you do not seem to have anything like that.
You really will be 100% on the hook for care for your DD, housekeeping, cleaning, etc--even more than now most likely, and you will be doing it after a long day of work and on little income so no extra money to jut eat out if you are too tired, etc.
A few thoughts about the convention (and I am not saying it is okay for you to need to ask permission to go places):
1. is it a multi day event that requires a hotel, etc? If so, is it typical for your DH to spend money like that for just himself and there is a double standard? Or, is money for hotels, etc generally always used just for the family so it feels like you are taking away from together time in order to pay for this? Maybe it is more an issue of looking at the spending and how to make it fit the budget and fit still allowing time for the both of you (or all of you) to get away-
or
2. could it be that your DH is feeling afraid or unable to care for your daughter for a longer time on his own? Yes, he should already be comfortable with this--but if he is not, perhaps talking about it and then helping him learn so that by the time the conference gets here he is not worried is a better approach than just throwing this out there and leaving him to cope with it on his own

Of course, both of those could be totally off base--but I think there are many reasons that your DH may be opposed to you going to the conference which do not boil down to simply being controlling and not wanting you to have fun, and it would be worthwhile to think about other possibilities and then try to resolve those issues.