I work with a girl who is twenty years my junior. I've told her that I can't stop thinking about her. I can't sleep, have no appetite. I'm not bad looking, I'm financially secure.
From what co-workers tell me, she only dates deadbeats and jail birds. Why can't I stop
Thinking about her?

The obsession is a problem regardless of age. I suggest you seeking some counseling for yourself. And if you're still interested in her once you are in a healthy place mentally, then look for advice. But for now, best to focus on you rather than try to start a relaionship

And even once you're in a healthy place mentally. There's a big difference if 20 years your junior puts her at 15 and you at 35 (or her anywhere under 18) vs she's 30 and you're 50.

What are the actual ages please?

If she's under 18, then stay away from her. Keep out of contact so that you don't end up in jail.

If she's over 18, remember, you already told her how you feel. So the ball is on her side of the court. If she feels the same way, she will let you know. If she doesn't, then find someone else.

It's not so much an age difference as it is a maturity difference. If you're 40 and she's 20 that's a little weird- she's still a young adult, 40 and 60 however, is a different story. This would depend on your actual age.

My opinion -
For a teenager, every year of age difference is huge. I wouldn't want my son dating anyone more than two years older or younger than himself in high school and college - life experiences are too important and drastically different at different ages in this phase of life.

In the twenties, I think more than 5 years makes it difficult to relate as one person has been on their own for several years, the other still has one foot in their parents' house and rules and world. There are big financial differences as well.

When the younger person is in their thirties, I personally think that problems are more likely to arise the closer to and beyond ten years you go. Both are certainly legal, but again, you are at different phases in life. The thirty-something wants kids, the forty-something doesn't, and so on. By the time the younger person is in their forties, I think it's a matter of preference. I am 45 and would date someone up to ten years older than me. More than that and they are about ready for retirement when I'm looking at another 27 years of working.

Twenty years is really pushing it. As others have pointed out, your obsession with someone who is not in any way reciprocating your interest, is excessive. The fact she's someone you work with puts you in jeopordy of sexual harassment charges if you act on any of your feelings at all. You've shared your feelings, she didn't respond, move on.

The fact you are so fixated probably has less to do with her and who she is than it does with your own lonliness. You probably do need to start dating, but someone your own age, or within a few years of your own age. If you're divorced, perhaps you'd relate better to a woman who's had the same experience. If you havce children, a woman who also has children could likely better relate to your responsibilities, and so on. Not always, but as a general guideline I think this is true.

My parents r 15 years apart in age

Yes, if one is 30 and the other is 15 we have some serious issues,

But the age difference is not even what I would worry about , the big no-no is the phase where they "work together" and how he seems obsessed by her and they have not even dated.

I am sure you have scared her, surprised she has not got a restraining order or something. You did not ask her out for coffee but told her you could not stop thinking about her. Came on way way to strong.

Next she may not like older men, some women do, or you may remind her of her dad and she things that is sick.

So now every time you look at her, you will start to feel rejected or get more obsessed.