I have very sensitive teeth is the pressure too strong?

It is possible it does take some getting used to if your gums are bleeding you need to use a periodontal mouthwash

for at least a month and brush your gums.

But, i still recommend that novices start adapting from the low gear

How often do I need to charge it? I'm worried that it's draining too fast.

I've had mine 3 weeks and used it daily. I see the seller suggests charging once every 2 weeks, you can use that as a reference

Is it portable? I travel a lot for business or pleasure.

For me, it's easy to carry, the tank is smaller for traveling, and the 200ml tank is enough for most people, whereas the 300ml tank of other flossers is too big to carry.

I see that there is a Custom mode, how do I use it?

The customization mode has a total of 8 gears, so you can easily find the gear that suits you.

1- turn on unit

2- hold mode button until "CH" "Custom" light flashes

3- each short press of mode button will change custom setting pressure.

Does the package include the charging adapter?

No. But I think it doesn't matter, because everyone has tons of adapters at home to use, It is enough to get a cable, which can be compatible with my mobile phone adapter and computer USB socket, It's convenient for charging.