The way I see it, there are 3 main factors that keep you stuck in bad habits like this:
• Autopiloting too much (so you use the habitual option without considering others and without considering why you're using it).
• Not being comfortable with executing the alternative options (so it becomes easier to just fall back on the habitual option under stress).
• Not understanding how the alternative options work (so you can't see the situations to apply them very well, and they aren't effective enough to make them worth applying much).
The other posters have already covered autopiloting; basically you just think and stop yourself from doing it.
You'd be surprised at how much getting comfortable with your techskill helps you break out of bad habits; once you have good control over your execution, dash attack loses its advantage of being easier to do than the other options and so becomes less appealing relative to them. The same applies to improving your punish game in general; if you can comfortably and effectively convert off a grab (for example) then you no longer benefit as much from using the "easy" combo starter in dash attack, because your alternative options now have a good enough reward to make them viable.
In particular, make sure you can comfortably and consistently wavedash out of dash-dance (at the angle that you want), so that you're not eliminating most of your grounded options every time you enter dash (most importantly: you keep your down tilt, and can stop moving without having to shield or jump). The more options you have that aren't dash attack the less you'll need/want to use dash attack.
Only dash attack when you actually want to dash attack, not just because it's easy to do and works sometimes. Think about what you expect to get out of it and what the alternatives are. Watch videos and look for how/when good players use the move. Dash Attack in neutral is a high-risk high-reward option, which only really works when you have a hard read on the opponent's movement/position, or when you can mix it up with other options to make it hard to punish consistently. Look at these clips for demonstration of good dash attack usage:
Low Tier City 2 Pools - Mew2King vs K9:
https://www.youtube.com/watch?v=voV1LSDYgXI&t=1m36s
Here, M2K sees K9 jump towards center stage because he's trapped by the ledge and wants to regain stage control. The dash attack is placed to intercept his landing; to avoid that, he would have to either fall (if M2K's reaction was faster then it would've been necessary to fastfall) and immediately shield/crouch/spotdodge, pull back so he lands on the edge of the platform, or double jump. While all of those options avoid the dash attack, the whole reason K9 is jumping towards center stage is to regain good position and reset to neutral, which is why he immediately starts dashing after he lands; he doesn't want to use those options, since he doesn't want to be stuck in shield/on a platform/above Marth with no jump. Here, dash attack covers the best and most likely option and ends up converting into a strong combo (and indirectly into a kill).
Had K9 have done any of the options besides landing and dashing away then the dash attack could've been punished or at least avoided, but those options aren't generally as strong as landing and dashing away; they're good because they beat the dash attack, but in order for them to be effective you have to read the dash attack (or something which covers similar options such as grabbing the landing), and if the Marth doesn't dash attack then they're significantly worse options by comparison.
If K9 had opted to land on the right platform (either by drifting or by double jumping), then he can probably punish the dash attack with a run off aerial from the platform before M2K can get his shield up or dash away. However, if he does that and M2K doesn't dash attack, then he puts himself in a bad position (above Marth on a platform; which is still better than being right by the ledge, at least) where M2K can continue to apply pressure. Double jumping to the top platform would be safe unless it gets hard read with a fulljump aerial, but leaves him unable to punish anything, and he'd still have to get down. If K9 landed and shielded or whatever, then ends up on the ground in center stage and he would be able to punish the dash attack; but then he places himself in a mixup between various other alternatives to dash attack that also cover his falling/landing, such as grabbing his landing (need to dash away, roll, attack, or spotdodge to avoid; all baitable/punishable by dash-dancing and adjusting the grab timing). Whether he double jumps or not, he can be intercepted in the air by an aerial, and needs to double jump out in the correct direction (depending on which way M2K jumps/drifts) or trade an aerial to avoid it.
All of these mixups are similarly punishing to the dash attack, and K9 can't cover all of them at once; this makes the dash attack relatively safer, since if he wants to punish it then he has to place himself in danger of the alternative options/mixups.
EXP 2015 Grand Finals - Mew2King vs SFAT:
https://www.youtube.com/watch?v=Xe_9LH4w86I&t=17m5s
Here, M2K is on the ledge and SFAT is trying to maintain his advantage until he can get him into an edgeguard. SFAT safely dash-dances outside the range of Marth's attacks from the ledge (ledgehop nair and fair) and tries to bait them (or other ledge options such as waveland grab). M2K whiffs a ledgehop fair, and SFAT continues to dash-dance a moderate distance away from M2K (outside of the range of Marth's standing and wavedash forward options). SFAT doesn't want to go in immediately after the fair, because Marth still has quite a few options (such as grab, spotdoge, and up tilt), all of which could end up getting him getting punished and put offstage. He also doesn't want to move further back than he is, since that would give M2K too much space and relieve the pressure of being by the ledge.
This is why the dash attack works so well; he can't dash back to avoid it without giving up his stage control, and he can't shield or crouch without giving up his movement options and attacks (and thereby giving up a lot of his pressure). Like the first example, in order to beat the dash attack SFAT must relinquish some of his coverage of the opponent's other options, and put himself at risk of being punished by things which aren't dash attack.
Because Marth is by the ledge and at relatively high damage (beyond the effective ASDI down threshold for most attacks), the dash attack becomes a much more reasonable risk to take. When you're on your last stock with a damage deficit, without crouch-cancel, and in a bad position, then you need to make something happen by going for high-risk high-reward plays; trying to play safe and going for a series of low-risk options is probably going to turn out in the opponent's favour, since they need to win significantly fewer interactions to beat you (all they have to do is get a hit or two and an easy edgeguard, but you have to outplay them repeatedly to reach the same point). Resetting to neutral (with a roll, etc.) is no longer as effective, and your options are limited, so the risk-reward ratio of dash attack becomes relatively better.
Just dash>down tilt is nice
You mean run cancel to down tilt; you can't down tilt out of a dash.
