A long time ago, I used to be a model,
Even though I was a bit thick boned,
I'd dress in casual clothes like everyone else,
I could have been a thousand different things, I was way ahead of my time,
It pains me to think I should have been a singer instead,
Nowadays, I'm not as mobile as I used to be,
People treat me like I'm a dinosaur,
And my bills are really adding up,
I'm not fooling myself, I don't want your pity,
At least my immune system hasn't failed me yet,
Who / what am I?
You could be
the letter T
A long time ago, I used to be a model,
Ford Model T
Even though I was a bit thick boned,
T-bone steak
I'd dress in casual clothes like everyone else,
T-shirt
I could have been a thousand different things, I was way ahead of my time,
T-1000, from the Terminator series (from OP)
It pains me to think I should have been a singer instead,
T-Pain
Nowadays, I'm not as mobile as I used to be,
T-Mobile
People treat me like I'm a dinosaur,
T-Rex
And my bills are really adding up,
T-bill i.e. Treasury bill
I'm not fooling myself, I don't want your pity,
Mr. T’s “I pity the fool” (from OP)
At least my immune system hasn't failed me yet,
T cell
