Reason for illegal target is that who ever your attacking is targeting themselves or another player.
If your in a group profit ui ftw. the Quick cast on the group window gives the options as a healer for your basic spells. It casts them on you while letting you keep your current target. Profit also has the nifty little player window witch allows you to cure your self quick fast. (Be cool if profit added the same group quick cast buttons to the player for pvp but i don't think he plays on the pvp server )
However if you like to solo (i do) A couple quick macros for your self are nice.
Just open up the macro tab Drag your fav heal spell in the window in the option for a target writ in your own name properly capitalized. That will cast the heal on your self while letting you keep your current target.
Also of note You can do two spells in a row. My self i like Cure with a small heal and cure with a big heal. But it varies with each class
Shaylen 80 inquis Storm Rising Nagafen
(anyone know if you can change your log in name here? haven't played izeals in a coons age)