You're just lonely sweetie. You look for love in the next nice
person you meet, and this man was probably there for you in your
rebound phase, where the temptation to feel the love and security
of having someone by your side is at its zenith. That is also,
unfortunately, the wrong time to be getting involved, not so much
because its too fast, but your mind isn't set straight, because all
it focuses on is company and love, wherever it comes from.
This man is a jackass, and I advise you to stop seeing him because
to me, it seems he thinks you're easy and takes advantage of the
situation to get the best of both worlds. One from you, the other
from the other girl involved.
Hang out with your friends more and do take care of yourself, and
don't be pessimistic. Lots of things happen to lots of people
around, be happy yours isn't the worst. Stay strong, do not falter,
and find within this bad experience what lesson is to be learnt.
Then follow it and move on. You can do it. 
