“I've never been able to stick to a diet.”

You're talking about dieting with a coworker at lunch. You've tried to diet several times in the past, but you gave up too quickly. You say:

I've never been able to stick to a diet.

I've never been able to (do something)

This phrase can be used when you tried to do something several times in the past, but didn't succeed.

stick to (something)

To "stick to" something means to continue to do it, even if it's difficult.

You can "stick to" a diet, of course. Other things that people try to "stick to" include:

• stick to a plan
• stick to your promise
• stick to the script (this means to follow the plan that was discussed at the beginning of a project)