Once you choose to be UBER then you are UBER from then on.
I forget where that choice occurs, but it should show up every time you use 100k multiplier until you choose it
But you are incorrect in thinking that a second UBER Star is related to using over a 100,000 multiplier a second time, third time... That would be a boring use of something cool like UBER mode.
To see what Uber Stars you are working on go to your Profile page and click the 'show uber details'.
The 19 Uber Stars, perhaps a half a dozen you are currently working on are related to using x number of monkey breaks, losing x number of daily hammers, winning x number of stars, winning x number of nothings... where x is a ridiculous thing like 10 million...

p.s. if I could have gone into UBER mode the day I started the game, then I might have 4 Stars completed by the time I got level 24 and all the bonus stages completed. They are ridiculous and only people constantly using 2 billion multipliers have anything like all 19 stars done.

You have not lost any Stars. Uber Stars ATR NOT what you think. Re-read my last one. As for where the 'activation' for Uber Mode is located, I seem to remember it as being quite unmistakably highly visible, wherever it was. The 'get hammers' page sounds right.
You should be aware that the 100,000 or higher multiplier MUST be pure and natural. No doubling from a Silver Egg, no 2x or 4x from Buffs, 100k or higher all on its own. An d I think a popup will occur the first time you do it so you don't even have to go to the get hammers (or wherever).

1. You should go to your Profile to make sure you have not actually activated Uber mode. Right below your Profile&Statistics box and above your Trophies you might have:
Uber-Mode Power Star Summary (What's this?)
Show Uber-Mode Details

2. When I show my details here are the 5 Stars that it's taken me TWO YEARS to get, and some others of the 19 total that I'm currently working on and will probably never get:
You have already earned the following Power Stars:
Monkey Feeder:
Use 1,000 Banana Tokens (since enabling Uber Mode)
Monkey Briber:
Use 5,000 Banana Tokens (since enabling Uber Mode)
Starsies:
Win 100,000 stars (since enabling Uber Mode)
Lost Hammers:
Lose 100,000 daily hammers to the daily reset (since enabling Uber Mode)
Uber Mode:
Enable Uber Mode.

Here are some other Power Stars you could win:
Monkey BFF:
Use 10,000 Banana Tokens (since enabling Uber Mode)
9,332 / 10,000
Extreme Multiplier:
Use 10,000,000,000x worth of multipliers (since enabling Uber Mode)
3,663,012 / 10,000,000,000
Extreme Scorer:
Earn 10,000,000,000,000 score
42,512,737,474 / 10,000,000,000,000
Startastic:
Win 1,000,000 stars (since enabling Uber Mode)
138,783 / 1,000,000
Buncha Nothing:
Win 1,000,000,000 Nothings (since enabling Uber Mode)
572,121 / 1,000,000,000
Bigger Loss:
Lose 10,000,000 daily hammers to the daily reset (since enabling Uber Mode)

others of the 19 are 10x higher most likely than some of these.