My answer got a bit long, and is primarily focused on some of the more generic skills for having difficult conversations. Since Avazula provided an excellent answer with some very specific suggestions for things you can do to move forward, I'll leave my answer with just the skills for how to have the conversation instead of what conversation to have.
To have the conversation you need to focus on
- Understanding where each other is coming from
- Finding a common ground to build a solution from
I've been in a very similar situation as your partner. The second girl that I ever dated, Julie, (who was the first person that I truly loved) died of cancer in her mid 20's about 2 years after we broke up. Two months later a good friend of mine's husband (who I also knew fairly well) died in a car wreck and a few months after that I lost a cousin with whom I was very close, also in a car wreck. I'd like to start my answer by walking you through what your partner is likely feeling, and then I'll get into how you can start the conversation about it.
What she is likely going through
When Julie died, she was the first person my age that I'd ever known who died. Along with my grief, there was also a great deal of shock as I was forced to really confront my own mortality for the first time. That shock overloaded my emotions and as a result it took me much longer to get over my grief than normal.
The other major thing that happened, was that Julie's death resurfaced a lot of my romantic feelings for her. When someone dies, we often remember the good things about them and brush past the bad. My relationship with Julie ended pretty amicably, and I'd seen her a few times after the breakup and we were still friendly, so it was fairly easy to remember the best parts of our relationship. Reminiscing about my relationship with Julie, combined with my already overwhelmed emotions and made me long for a return to our relationship even though we had broken up 2 years prior and I had gotten over her in that time. It stings quite a bit having these feelings come back up when you know that the finality of the other person's death means that there is no way your feelings can be reciprocated. You mentioned that your partner realized that she loved her ex when she reconnected with him, so I would think that she is experiencing the pain of unrequitable feelings even more strongly than I did.
A quick note about multiple deaths close together
I definitely would not discount the point you made about the other people she's lost. When you start to lose multiple people in a short period of time, the grief compounds quickly and it is easy to get overwhelmed by your feelings. By the time I'd experienced a third death in about a 6 month span, I couldn't process my feelings at all and I completely shut down.
How to have the conversation
I'm not going to lie, this conversation will probably be a very difficult one to have, but it is important for your relationship that you do have it. When I found out that Julie had cancer I was dating Lucy and we never really talked about what I was experiencing. In dealing with my grief, I was not a good boyfriend, and our relationship ended quite poorly.
How can I have a conversation about this and what things should I consider so that we can both understand each other's feelings properly?
You're off to a good start because you already understand that the first thing you need to accomplish is to understand each other's feelings. I recently had training at work on how to have difficult conversations, and the thing that we talked about as being most important is to get all of your emotions on the table so that you can understand each other. One of the best ways to do this as we discussed in my training was to ask open ended questions. For example, when she says that she misses him, you can ask her what she misses about him. Just have an open and honest dialogue about her relationship with him. Talking through her relationship with him will help her to get her emotions out in the open for you to understand them.
At some point during this conversation, you'll also have to get your feelings out there. During this part of the conversation you need to focus on how you feel rather than how she is acting. A good way to do this is using "I" language instead of "you" language.
“I” language is often the best way to communicate with someone during a conflict. “You” language can be construed as blaming others and placing the emphasis on what they are doing wrong. “I” language consists of taking responsibilities for your own opinions and emotions and gives you more control over your emotions.
When you are explaining your side of the issue, focus the conversation on your feelings and minimize talking about her actions so as to avoid casting blame. Ultimately, she can't stop herself from feeling the emotions that she does, and saying things like "I have trouble not feeling hurt that you still have feelings for him" will soften the impact of telling her that her actions hurt you. Unfortunately, you won't be able to completely take away the discomfort that the two of you are going to have during this conversation, but hopefully you can keep it to a minimum.
Another useful technique that we discussed in my training was to create contrasts as a way to clarify where you stand. This is particularly useful if anything that you've said upsets her. As an example, say that you tell her that you feel hurt and she responds defensively.
You: I have trouble not feeling hurt that you still have feelings for him"
Her: Well I can't help what I feel.
You can respond by contrasting the incorrect meaning that she gathered with what your actual intention was.
I don't mean to imply that it's bad for you to have the feelings you have. What I do mean is that I also have feelings that I can't make go away.
As you are talking, make sure to express your goals for the conversation. For example, "I want our relationship to work, and ..." The goal is to establish some sort of common ground for the two of you.
In rhetoric and communication, common ground is a basis of mutual interest or agreement that's found or established in the course of an argument.
Finding common ground is an essential aspect of conflict resolution and a key to ending disputes peacefully.
Even though the two of you are not currently in an argument, finding common ground is important because it can help you avoid having this conversation turn into an argument. If the two of you can agree that you are working towards a common goal of having a healthy relationship, that will help shape any decisions you have to make about how you move forward.