While you can't directly control the call, there are certain things you can do to help generate the readiness sooner.
A great step is to work with your mind by identifying and learning to disbelieve anxiety-provoking thoughts and attitudes. It's not generally possible to totally control one's thoughts, because undesired thoughts pop up before you have the chance to stop them. In fact, it's usually unfeasible to try to control thoughts at all in hyperspace because the healthily stubborn attitude that can enable a non-tripper to refuse to succumb to negativity can easily be thwarted by the negativity itself in hyperspace. It turns into resistance, which is futile because you can't control what's going on. So then it turns to panic, or despair, or frustration, or some other unpleasant emotion which is heightened by the intensity of the experience. I bet we're all familiar with that sort of thing happening (even if your experience is not exactly the same). My point is that it usually starts out as "not horrible" but under the influence of a psychedelic it escalates because of the tendency to get stuck in mind-loops.
But you can absolutely control thoughts indirectly! You can condition your mind to produce fewer unpleasant thoughts by finding out what underlying attitudes are making you think that way. If you correct the distorted thoughts that are making you feel bad on Monday, you're likely to feel better the whole week.
So what happens in your ordinary life that causes anxiety of this type? Do you have problems with body image? Or perhaps worrying about your body? Or maybe worry in general? Is there something that you walk around hoping doesn't happen? Or fearing that it is happening?
I say this because with the exception of cases where you experience physical discomfort in the body (such as ayahuasca nausea, or burning yourself with a hot flame when smoking DMT), you can safely presume that negative experiences are psychological in origin. Why did you get stuck in the negative mind state? That sort of thing is almost always caused by distorted thoughts.
If you didn't have any distorted thoughts or beliefs, it would of course still be possible to be uncomfortable. Everyone knows that there are some things about life that honestly do suck -- but outside of situations where your life or the life of someone you care dearly about is in danger, when is it really appropriate to be panicked to the degree you were?
So the fact that panic (outside of life threatening situations) is caused by distorted beliefs is really useful to a psychonaut. Getting caught in a negative thought loop while tripping generally means that you have a strong enough habit in daily life of having that thought (or something like it) and believing that it's true. In other words, you may be spending enough time in ordinary life "practicing" anxiety that it turns into a habit. In fact, I'm sure you are, since you say "the anxiety is unbearable." It's easier for that habit to manifest in a major way while you're in hyperspace since you can't directly control thoughts then.
So find out what's going on with the thoughts that are triggering this anxiety. If you don't know how to do that, I can give you some excellent resources. Just let me know.
So that's half the equation for quickening your return to pleasant tripping. (Or at least, to not having any horrortrips.) What's the other half? It's still conditioning, but at a much simpler level. As you've recognized, you formed a new association that says DMT = terror. Working with your thoughts is great, and has a wonderful effect on mood (being sufficient to reverse even severe clinical depression sometimes, and usually mild to moderate depression). But it won't fix the conditioning, because the conditioned response is programmed into the brain at a much more basic level than thoughts are operating at.
How do you reverse a conditioned anxiety response? Gradual exposure. I bet that right now, even thinking about breaking through to hyperspace makes you nervous. So think about hyperspace a lot. Get yourself used to the idea of blasting off again, even if you don't feel less anxious now. The goal in the beginning isn't necessarily to feel totally non-anxious about DMT. It's to get used to thinking about it, and develop some amount of comfort with the thought. (Comfort and mild anxiety can co-exist, unlike comfort and severe panic. It's the severe panic we need to prevent, because mild anxiety does not ruin a trip unless it escalates.)
Next step (or first step if you aren't willing to spend the time thinking about DMT first): Threshold doses. It's the same idea -- go ahead and do some threshold doses, even if you feel a bit anxious. Just get familiar with it. Do this at times when you're already feeling pretty good, or at least feeling nice. Don't do this when you're already uncomfortable about something other than the thought of a threshold dose of DMT.
Then increase the dose gradually over time. How fast? Fast enough that you can notice the progress happening, but not fast enough for your anxiety to go past "mild" into "moderate" or "severe."
By the time you're at medium to high sub-breakthrough doses, you'll notice that you don't have anxiety with threshold and low doses. "What if I don't?" You will. This is how re-conditioning works. It's really hard to screw it up as long as you follow the guideline, "Don't freak yourself out badly. Only let yourself get mildly anxious." The whole idea is to see that even if you're a bit anxious, you're still okay. This breaks the pattern of starting with mild anxiety and escalating into panic. Unlike trying to voluntarily control thoughts, this does work when you're tripping hard because psychedelics don't interfere much with reflexive associations like this. If anything, they strengthen them. So it would only take a few good trips to install new conditioning.
I can assure you of the effectiveness of this process. It's empirically validated, and shrinks use things like this all the time to help people deal with phobias and anxiety disorders all the time, including the severe ones. So these techniques are the "big guns" for dealing with anxiety, and you may need the "big guns" to recondition yourself after a horrortrip. It worked for me, and I had a 4-hour long severe horrortrip followed by a string of ordinary bad trips.
I'm a bit sleep deprived, so parts of this may not make sense. Let me know if something's confusing and I'll explain it better.
Now that I've had a chance to explore it somewhat I haven't really felt it for several months. 
