I would first bring your set of questions to a priest or spiritual director.
We are not priests and any advice or suggestions we give should not override the counsel of a faithful Catholic priest who may learn some nuances about your situation that you have not shared with us.
The only thing I can share is my personal opinion/viewpoint of what you have shared with us.
When the Church requires or prohibits something, it is not because She is trying to just be mean or lacks compassion. Anything the Church requires or prohibits is done for one reason:
To remain faithful to the Teachings of Jesus, our Savior.
That said, whether the married man's marriage is valid or not, is not your concern. In the eyes of the Church, until a declaration of nullity is received, he is married and you shouldn't even think of falling in love with a married man. That is adultery which breaks the sixth commandment.
I know I'm just going to get hurt in the end.
You're right!, because you are setting yourself up to get hurt.
Christian Marriage is a life-time commitment between one single, baptized man and one single, baptized woman which is open to life. This idea doesn't come from any stranger, but from Jesus, our God-Man, Himself.
Listen to our Savior.
On a personal note, after reading questions and answers to situations dealing with marriages, annulments, and divorces for over ten years, one thing I can say with confidence as a single Catholic:
- If you don't follow the Church rules on marriage, you can really mess up your life.
Marriage requirements and/or prohibitions our for our good and for our eternal salvation.
And as my mother constantly reminded me:
Michael, It's a lifetime commitment.
Besides the issue of dating a married man, let me ask you:
Can you get to know someone, as you said, in a few months, then make a lifetime commitment? <I don't think so.>
If this man, in the future, does receive an annulment and is free to marry again, if he marries again, whose to say, after three years of marriage, he doesn't choose someone else, like he told you!
I hope this helps,