Here's my insight on the matter:

I've had my share of heartbreaks, in which I've learned a lot and openly share to others who are willing to listen and learn. Going a bit psycho over a messy break up is kinda okay for me especially if you've invested so much in the relationship, to the point of lowering your standards and accepting all the crap that he's putting you through. Thinking that with all the unconditional love you're giving him, he's gonna change.

If you're not married to this guy, I can tell THERE IS LITTLE TO NO HOPE OF YOU CHANGING HIM.

Going psycho is a loyal girl's effect to a bad break-up. Bad break-ups may include cheating, flirting with other girls, physical abuse, and hurtful words or anything really that would not make you and him end up as friends. 

So yes, if you were a loyal girl and just got dumped by the guy you thought was the one because the girl he found is willing to give in to the things you said you would never do. By all means, YES! be a psycho ex girlfriend for a while.

But being this psycho must end at some point, because it will only look pathetic if you continue to be this person in an extended period of time. One must learn to move on at some point, accept the fact that, that relationship didn't work. Just think of this chapter in your life as a learning experience, you now know what to look for in someone you would want to be with for the rest of your life. Be happy that you found out now because it would really suck if he does this to you when you're married and expecting a baby on the way.

So there, be a psycho. Get it off your system. But in the process, let go of all the anger and forgive.