I am going to give the women names, because they are easier to work with than just initials.
I have been living with a wonderful woman, Kim, for about a year and a half. I want to keep the relationship and living arrangements I have.
But!
I recently admitted to myself that I am erotically attached in my heart to a woman with whom I have corresponded for 17 years and never met. Let's call her Friend. Kim doesn't know about my correspondence with and feelings for Friend. I am reluctant to tell her because Kim is a very insecure woman, romantically. I've never told Friend exactly what my feelings for her are, mostly because she's married, and I don't want to cause a fuss.
Is this a poly conundrum? Am I just a horn dog? I'm feeling rather bottled up and am pondering what I might do about it all.
I feel bottled up because I think I should share my feelings about Friend, which are long-standing, but I don't trust Kim not to take this personally.
You are afraid of hurting Kim, and...
I feel like I am cheating.
As far as Friend goes, long ago she stated that her husband thought of me as her boyfriend. I was too afraid at the time to explore the ramifications of that. But I have wanted to engage in online sex play with Friend for a long time.
First you said you have only recently found yourself feeling "erotically attached" to Friend. But now you say you've wanted to do cybersex with her for a long time. You think maybe her husband would be OK with it, since he calls you her boyfriend, but you've never dared to explore the potential more deeply.
For all you know, Friend and her husband may be open to polyamory? You won't know unless you ask.
On a tangent, before I met Kim, I was on OKCupid and discovered a polymorphism friend there...
Polyamorous?
I was attracted to her and immediately asked her out. It didn't bother me in the least that she had a spouse. Unfortunately, she turned me down.
You might be poly. You don't feel sexual jealousy. But your current partner, Kim, and you are practicing monogamy. And it seems she still feels insecure in your relationship. Do you think she suspects you aren't really on board with monogamy? Do you want to mislead her forever? Do you want to live mono forever with Kim, when you long to explore more with Friend?
Being horny for multiple people is (usually) part of polyamory. Having feelings for multiple people is required. I think, unlike GalaGirl, that it is a bit strange Kim doesn't know about your long distance good friend whom you love and desire.
If you feel bottled up, or like you're not being true to your poly nature, you have a few choices.
Continue as you are, bottled up, talking to Friend secretly, not telling Kim, not asking Friend is she is open to cybersex (or even meeting you irl).
Tell Kim you have a crush on a good friend, whom you've talked to for 17 years, but it's never gone anywhere. It's just a crush.
Ask Friend if she is open to cybersex. Ask her if her husband and she have an open relationship.
Tell Kim you want to deepen your relationship with Friend. You love 2 women.
You might lose Kim. But you'd be true to yourself. And maybe you'd find out you can have a poly relationship with Friend. And you can go on to not date people who aren't open to polyamory. Let them know about your online love interest.
Or, you could admit that having a crush on your online friend isn't helping your irl relationship develop in trust. Stop talking to Friend. Let the feelings subside. It might take years, but they will. There might always be a little spark, but it won't feel so urgent and tempting and inappropriate.
Is it that you wish you could talk to K about these things, but don't feel safe doing it because she will make it be about her and her insecure stuff? Rather than listening to you sharing and let it be about you and your stuff? That's a different problem than simply having feelings/admiration for the married pen pal F. 
