Plumbing – Looking for a volume control valve for an under the sink water purifier

Since I am into plumbing these days I would like to fix one problem that has been pestering us for quite a while now. I installed an under the sing water purifier (This Guy, RainFresh SST Mode). The water is coming out way too fast, some times splashing over the counter if you fully open the tap (it has its own tap)

Is there any way to control the volume for this thing ?(I guess this is the problem) Opening the tap more or less seems to do the thing but you need to pay attention and the pressure is just to much and you can't always get it right so the water splashes all over the place.

Here is the diagram, the only difference in my case is that I have a shutoff valve on the 1/2" pipe and then right on top of it I have a 5/8×3/8×5/8 compression tee that branches out the 3/8 line for the water purifier and also goes to the regular cold water tap using a hose

So I guess I need something that can be either attached inline to the 3/8 pipe that feeds the purifier or replace the T with one that has a turn off valve on the 3/8 line, I just can't find the part