I have just decided on a whim that I no longer want to support the dairy industry and cut it out of my diet cold turkey. I've already trialed a run and I felt great but it was so difficult. Never realized how much dairy was in my diet in basically everything I ate and it wasn't long before I ran out of things to eat that didn't contain it. Even subjected myself to eating cereal with water which was extremely unpleasant. It's been hard but I know you're the community to ask for help because you've probably went through this sort of thing before.
There's a ton of vegan alternatives to milk products! If you can, give a few a shot at a time. Soy, oat, almond milk.
Vegan butter can be very convincing or just taste like margarine, which is also fine.
Vegan cheese is generally unconvincing, so prepare yourself for that. If someone says they eat a lot of dairy, I assume it's mostly cheese. I would recommend instead finding vegan foods that hit a similar spot. Hummus is nutty, salty, tangy - get or make some good hummus.
Some brands of vegan cream cheese are very good and go great on a bagel. Some are just okay but taste great when you add some other things like fresh-cracked pepper and a thin-sliced veggie.
This also might be a good time to find some new staples! A nice tangy bean salad is very nutritious, easy to make, and will last several days. Tabouleh is healthy and delicious. Basically, some people risk having bad nutrition when they go vegan because they simply drop meat and dairy without compensating, so you want to avoid that.