I'm 61 and my girlfriend is 59. We've been together for just over six months and are very much in love. However, her husband died almost 2 and a half years ago of a sudden heart attack at age 58. She continues to talk about this guy.

For example, every time I give an opinion on something, she tells me what his opinion was on it. She tells me she would give anything to have him back even though she loves things about me that she didn't get from him. She tells me that I should stop being jealous of a dead person. Other than that, we have an excellent relationship and never fight about anything because we both have the ability to talk everything out without anger.

I would think that two and a half years is long enough to stop constantly bringing him up.

I agree, but she's not the one here asking for help, so the choices are all yours. If you can't gently talk her out of it, or get her to join a bereavement group, or some other outlet for her feelings, then you have to decide what YOU are going to do.
I would try a little speech when it isn't happening, a sort of 'I have something important to say.' And then take her hand and tell her how it deflates you, and even hurts, and you feel like he is an elephant sitting between you, and you hope she can talk about it with someone else or a group. If that doesn't work (either then or in practice), then I would suggest walking out of the room each time she does it. Just say 'I'm going for a drive' or to bed or putter in the yard or garage. Nothing bitter or angry, just looking out for yourself. There's only so much you have to sacrifice for this, and you've done it.

Thank you for your input. I'm at work at the moment so I do not have the time to respond more fully... but I will in a bit. I just need to make sure that I'm not being selfish or stupid about this.

Strictly speaking you would be considered jealous of her (e.g. losing her to someone else, or not as good as someone else) and envious of him, but jealousy has lost it's dictionary meaning.
I mention that because she needs to realize that you aren't envious of him (unless you wish you were dead) but that you are jealous of HER. Her dwelling on someone else.

I lived with my father for several years before he died and he was just so sweet, everyone liked him, in fact he was so loved that it felt like I was being compared to him when they gushed about him to me. But I miss him so much and I think of him every time someone else is less than perfect. But I keep my mouth shut! They don't need to me to compare any more than I need to hear how wonderful he was. And that's nothing compared to having to hear about an ex husband. I don't understand why she doesn't understand what she is doing. Maybe you have a mutual friend or a sibling of hers or someone who can explain it to her. You could also tell her you need a vacation from it, and take a week off with some buddy.

It sounds like she hasn't finished grieving for him. Yes, encourage her to get grief counseling or join a grief support group. Maybe, if you are able, say things like, "He must have been a wonderful guy to have been with someone as sweet as you" or "And you are the lovable person you are today partly because of him." Encourage her to write down her good and funny and even not so good memories of him, perhaps in a blog or a special notebook. If he had a special interest or charity, go in with her to make a contribution to it in his name. In other words, do some small things to help her finish grieving.

WonderGirl has more positive, proactive suggestions than I do (always)!
Good ideas. I don't know if I could do them.

We've been together for just over six months

I would do nothing at all, just let her enjoy her still fresh memories of a past life, and understand her need to. The problem is you take it personally. Don't.

2 and a half years may be adequate for you but not her. Its only been 6 months for the two of you. More to learn and adjust to awaits you.

No one understands unless you have gone through it. I lost my first husband 5 years ago and I have been married since Sept. last year. One thing you have to remember is that a person who loses a spouse will always grieve. It doesn't get better, they learn to live with it. She doesn't love you any less then she did her first husband, she loves you differently then she did her late husband, she chose you We widows tend to share our memories and when we do all we want is for you to listen. I love my late husband and always will and I love my current husband. We can love two men. Is it fair to you?? Don't know, but all I can say is that allow her to be who she is, and be glad of the moments you have together and don't allow her late husband memories to destroy what you have today, sounds like a great relationship! Embrace it! Remember the past is the past, but the present is a "GIFT"!