My difficulties usually start when I can't walk, and I'm left, in place, to come up with something that doesn't (can't) move. That's another story.
Ah, ok, if that's your problem, maybe this will help...
I agree pretty much with everything the others have said so far. I would like to add one anecdote that helped me move past that barrier.
So I went to one of the local tango festivals about a year and a half ago, and one of the classes was called "Essential Steps". I was kind of curious on what steps might be considered "essential", and it was in the beginner track, so I tried it out. What the class ended up being about was
navigation. The teacher proposed 3 basic combinations.
1. Rockstep turn, starting with the left foot forward, then rotating the follower to the left. The goal is to be able to get a full 180 degrees from where you started. In the middle of the rockstep turn, the leader double steps in the middle, so you start facing forward, and end up doing ochos the opposite direction you started.
2. Simple quarter turn to the left, starting with a back ocho, ending with a forward ocho/cross, and walking out.
3. Ocho cortado, and variations involving continuing the turn to the right and ending with the cortado cross back to the left.
So we practiced these steps, and practiced them again, then worked it so we could go back and forth between the three. In particular, 1 and 2 tie well together. Then the teacher revealed that the point of the class was that the steps themselves weren't essential, but the idea of having a couple navigational movements to fall back on is great for a beginning leader. Blocked from the front? Start your rock turn and look to your left with the corner of your eye to see who is coming up behind. Someone racing up on you from the back? Quarter turn back around to the front. Danger to the left? Ocho cortado to protect and move your partner out of the way, then use the moment to take a quick peek behind and to the right.
Once you have a couple rotational movements you can stick together in a pinch, you will feel a LOT more comfortable navigating the floor and moving just in one place. You don't have to pick the three I mentioned, but they are good ones. I'd choose one turn to the left and one turn to the right at the very least. Get so you can them without even thinking. Then build off them. And perform them with different musicality.
There's always the possibility of getting those steps stuck in your head and getting in a rut... but at least then you're moving down the road.

Once you get up some momentum, you can hop out and get back on the improvisational pavement.