We talked for long about each others love life. She told me how her boyfriend helped her to abort some years ago.I told her I knew the story the day it happened and she got surprised. She can't leave her boyfriend because she fears that if she looses him, she might not get some other guy. She told me how lucky my present girlfriend is to have me. I told her how I had longed to have her as my girlfriend and she jumped to kiss me. I told her I don't mind what happened but she doesn't trust me. I also love her but she cannot leave her boyfriend.

The old can't thing

We can do anything we really want to do in this life.. she does not want to leave him because she does not really want to.
Simple as that
And you.. should be ashamed of yourself
Well both of you should be
You both have partners
Grow some B@@@# and tell your current relationships that you want other people
Don't string them along.
I don't feel sorry for you
I feel sorry for the poor people you are dragging along in this very young game.

Once you guys deal with your current relationships
Then you can work on other things
Until then.
Be a man and tell your girlfriend how you really feel

I wouldn't trust you either, as your proving to her, by words, and actions, you are a liar, and cheater.

You are both with other people so that makes both of you cheaters.

If you are unhappy in your current relationship,do the decent thing and break up with your girlfriend.

As stated by the above, this is a juvenile game and the proof is piling up against you: I wouldn't trust you either and frankly, I don't know how you can trust her.
You are both clearly in unhappy, dependent, unhealthy relationships and have found understanding in one another. Do the decent, right, just and logical thing and break up with your girlfriend. If she cannot break up with her boyfriend for you, she clearly does not want you as much as she says she does.