Why do I have to hold down the handle until my toilet completely flushes?

Incorrect flapper for the toilet. They used to be standard but now each manufacturer seem to have their own type. If you still have it, reinstall the old one.

this is not a water pressure problem in fact no pressure is actually needed when flushing...lol nice try here is the problem the float you know the big black ball in the toilet tank is is screwed on to snugly. this is how it works if you take that black float and turn it screw it on the armature arm that holds it the more you screw it up turd the end of the thread the less it releases the plunger flap on the bottom. this leave slack in the tension arm of the stopper plus near the top tube. so what you got to do is turn the black float either way don't know which position its in now but one way loosens the tension and the other tightens the tension if the float is screwed out to far then this tell the stopper on the main tube to stop filing the water pre maturely so when you flush it you got to hold it till the tank drains. so you probably will have to experiment screwing the float either way and test it accordingly i would start by screwing it in word first just little by little until the problem is resolved. this will fix your issue. good luck.

I must agree with Sensible. I have had this happen with two different toilets. In one case, I put the old one back in, and it worked. With the other, I bought a separate (soft) flapper valve at the hardware store, and that solved the problem.

Take some slack out of the chain and put the float closer to the flapper

the chain that is attached to the float is too long, open the top of the tank and move the retaining clip up closer to the handle or make the chain shorter

did you trim excess chain?

low water pressure