Yup, real force but cautious answer here because you don't want to be busting anything
It may be worth your while to take off both side fairings so you can get at the back of the headlight assembly properly, even remove the headlight assembly (two bolts on each side if I remember correctly). Turn it over or, if the wire spring clip that holds the bulb into the reflector will open with the adapter in place then take the bulb out with adapter and plug still attached to the harness so you can see what you are doing. A shot of W.D.40 or the like in the join between the adapter and the plug may help. A thin flat blade screwdriver between adapter and bulb may help. Lots of wiggling It doesn't matter if the adapter doesn't come off the plug, it's the bulb obviously, that has to come out of the adapter.
If you grease up the terminals before re assembly it won't be such a pig next time. Might be an idea to do the other headlight too while you've got it all apart.
Let us know how you get on. You aren't the first to come across this problem