How to make a character to NOT stop moving in direction until I press the opposite button?

I think I have made an error in my explanation - sorry.
What’s happening at the moment is that I press a key and the object starts moving which is great. It continues to move which is also great. However - it automatically starts to accelerate by itself. I would like to control when that acceleration happens myself.
That way I will have full control over it, to the point that I could even stop it moving altogether.

My apologies!
Would using the velocity be the key to this?