I am eighteen years old and I consider myself a self-sufficient and attractive young woman. I am a virgin, too. (When I refer to a "sexual relationship" from here, I mean oral sex.)
The reason why I'm writing here is because it seems that I always become the "other woman" in a relationship. It began in high school when my boyfriend of 2+ years and I broke up (our parents made us because they felt we were getting too physical, and in retrospect, they were right. I was a junior.) A month after we broke up, we continued to have sexual relations (oral sex, kissing, etc) and he continued to tell me he loved me even though he had a girlfriend. This unhealthy relationship continued in a physical manner until he turned 18 and in a mental manner until I left for college this fall. I still have strong feelings for this guy, but at this point I KNOW better about him.
This summer, I "saw" another guy, who was aged in his mid-20's, who was the brother of a friend I worked with. He had a long-term girlfriend and a history of cheating, but since we were not committed and I did not have strong feelings for him, the sexual relationship we had did not bother me. We also broke it off when I left for college.
I went 3 and a half hours away to go to college to get away from my less-than-enviable home life and also to restart where people did not know that I was so-and-so's "mistress," so to speak.
I swore I would never be the second woman again. I specifically avoided being near any other woman's boyfriend. I hang out with a small fraternity that two of my new male friends joined last semester. I met a guy there, let's call him John, who is a senior. He is tall and good-looking and has a very nice personality, but he also has a VERY sweet girlfriend. I am becoming a part of their non-official girl's program this semester and he is supposed to become my "big brother."
Along those lines, I began to flirt with him in what I percieved to be a HARMLESS manner. However, we kissed at a party one night and proceeded to go up to his room. He convinced me to try anal sex for the first time and while it was a wonderful experience, it has not repeated. Neither have we talked about it since.
"John" is not a "bad guy." I am not a "bad girl." My question is (and sorry for the long lead-up) WHY do I end up in these situations? It is almost like I am MORE CONTENT with "sharing" a man rather than getting my own. I have often told my friends in a joking manner that it is "easier" to be the mistress because if your lover makes you angry or vice versae, it is not your responsibility. If I don't want to be with my lover, he has a girlfriend who is supposed to be there for him.
I thought I saw someone who was a marriage counselor talking on here. I could use some advice from you, or anyone who has been there. Is there something emotionally wrong with me, or do I just keep getting in bad situations?

As soon as most women learn a guy has a girlfriend,
he is no longer attractive. It seems that as soon as
you see a man has a girlfriend, he becomes more attractive
to you.

You began to 'flirt' with him in 'harmless' manner.
No.
It wasn't harmless.
Because you wind up in his bed.
And it's pretty clear the 'harmless' 'flirting' came
after you knew he had a girlfriend.

I can tell you the problem, but it won't work unless
you see it yourself.

Many women have low self-images and compare
themselves to other women on a kind of....'am
I prettier than Linda?' "Am I sexier than Denise?"
And they go after Linda's or Denises boyfriend,
because any 'getting' of that boyfriend 'proves'
they are prettier or sexier.

So, here's where you are at this moment in your
life...a free blow job, and free anal sex.

So, you're not the mistress.
You are cheap sex.

Pretending you are a virgin because your
hymen is broken is only fooling yourself.
These guys are having a ball!
Imagine, get a girl to go down on you,
or to allow you to slam her in the ...
with no strings!

A lot of girls won't give blow jobs and
most of them won't even consider anal
sex.

So, it's back to Scarlett O'Hara...
a good girl to court and a bad girl
to fool with.

Subconsciously you have a very low opinion
of yourself. This is why you put yourself into
these situations.

Your self-esteem will be even lower, when
John's friends ask you for anal sex.