<human>: Can you provide a brief overview of the text?

I've been seeing my current girlfriend for 6 months now, and I'm really worried that emotions pertaining to my ex may pop up.
To put things in perspective, up until recently I have had a bit of a hero complex when it comes to relationships and my ex was in dire straights for most of our relationship. When my ex and I decided to end it, within two weeks of us breaking up she had (as well as her friends) messaged me to say that she had slept with at least one other guy because she "couldn't bare to think you [rei154] were my last", but had also contracted an STI off him. As well as this, her addiction to Marijuana grew stronger and it became harder to be around her, because at the time I still loved her. Over the next year, shit kept happening to her that was entirely preventable, but she refused to deal with the situations any differently to how she already was. Because of this, I decided to break all contact with her because it was royally messing up my life at the time.
Now I don't want this to turn into a rant about my ex (that would be fun for no one), so I'll stop short and say that I believe that she is a very good person, who was brought up and treated incredibly badly.
Fast forward to present, in current relationship (six months and counting!) with the complete opposite to my ex in most ways; doesn't drink much, no drugs, doesn't go clubbing often, etc, etc. Things could not be smoother in this relationship. The problem is, I'm finding it harder and harder to not think about my ex. I don't know why, she did so much to hurt me and we were such different people, yet I can't help but wonder if she is okay.
This hasn't changed my behaviour with my current girlfriend at all, but I'm worried it might soon, and I'm worried my feelings for my ex might come back.
<bot>: I'm still thinking about my fucked up ex and I'm scared it's going to ruin my current relationship, as well as cause me to slide back.