I don't know of anything for mustard akin to the chilli 'cures' such as yoghurt, milk, etc but as a general rule, vinegar knocks back the heat of mustard, water emphasises it.
Additionally, water-based will naturally lose its heat much faster than vinegar-based - which is why you generally make up mustard powder only 15 mins or so before you use it. Pure water/mustard mix will have noticeably lost its edge after only a day.
It might depend on how, or if, your mustard is preserved initially. If it was water-based then canned, then knocking it back with vinegar & a milder mustard powder would change the flavour profile but may also change its shelf-life. If it's vinegar-based initially, then I think your only option is to somehow dilute it. I would go with a milder home-made mustard.
It may depend on where in the world you live as to the availability of mustard powders. Here in the UK the only ubiquitous one is an English, Coleman's, which will certainly not reduce the burn at all & may actually increase it.
In general, darker colour mustards are milder, lighter are hotter. 'Fake' ready-made mustards like McDonald's use all kinds of fillers, bulkers & colourings to make a yellow 'mustard' with almost no taste of mustard at all, so don't let that fool you.
If you don't mind changing the flavour profile completely, then will mix with mayo - which, in effect, just dilutes the mustard. Honey will sweeten it & dilute somewhat. Sweet will mask the hot to an extent.