Not that I'm a sex therapist, but it could be any number of things.

1. It sounds like you've had lousy lovers in men. The rule is "Don't come up until you see the whites of her eyes!" So those instances had nothing to do with you, but them just being a terrible lay.
2. If you are taking any antidepressants, those can cause delayed or other orgasmic disorders. So instead of riding the wave after an orgasm, you'll get a "Hey, all done down here. Can we get nachos now?" So medication is something to check.
3. You said it's something for when you get bored. Mental arousal is very important to pushing up to and over that edge and the longer it goes, the more you program yourself. This is something a therapist can help with as I'm guessing if it's this, then you've having something similar to "performance anxiety" in men where they can't orgasm either.