I've had the chair seat problem in the past, so I'll tell you something simple that helped me: shifting into a slight two-point, and then sinking right back. It puts your leg into alignment if it was stuck forward before; if you sit back slowly enough you force your leg into place. Then you've just got to hold it. And repeat as often as necessary.
The thing is that it hurts until your muscle memory is strong enough for you to do it on your own. My knees naturally reach up and my legs naturally shoot straight out in front of me when I sit on a horse; it's taken years to overcome this. My tendons and muscles were screaming in agony until I learned to do it all automatically. For awhile literally all I did was practice my seat/position. It took far more than my trainer simply showing me a picture or placing my leg with her hands, we had to realign what seemed like every bone and cord in my leg!