For me, the jealousy would become overwhelming (to the point where I would build up these scenarios in my mind and convince myself that my gf was cheating on me). Once you get to that point, it's really hard (impossible for me) to go back. I would obsess over it, look for things to justify my thinking (and if you want to believe something bad enough, you can always find things to justify your reasoning).
The reason it would poison the relationship is that while my gf (and there were a few I went through this with) would be a 'normal', loving gf, and I would not return any of the love or emotions that a partner should. Instead, I would exhibit negativity and distrust. Once you lose trust, it's really hard to get it back. And the fact that I was losing trust over things I had come up with in my mind made it worse. It became all encompassing. I was a control freak.
It wasn't easy for me to move past that phase. I stopped dating for over a year and just worked on my issues with trust. So, understand that it's not going to change overnight. The best advice I can give you is two-fold:
Always keep open communications. With my current GF (of almost 5 years), we are open and honest about everything. She has an ex that still wants to be with her, but she has absolutely no interest in him, yet he will still show up at parties and talk to her. Since we've discussed the situation, and I fully trust that she's telling the truth, it doesn't bother me at all. If she is lying to me, then there's nothing I can do about it anyway, so there's no point in getting worked up over it. She's not lying, but that's the kind of attitude you need to develop.
Back to the intellectual part. You really have to believe that you only control yourself. Facing your fear is the best way to take away the power it has over you. If my gf did cheat on me, then the relationship would end. It would suck, and I would be hurt, but I would move on and still live my life. I wouldn't be the one embarrassed or humiliated, that would all fall on her. That's the mental approach you have to take. The whole "face your fear" thing was something I worked on with a therapist (that helped me get over the jealousy thing). It's like an exercise (mentally). It really helped me.
The bottom line is to be the best boyfriend/husband you can be. Don't take your partner for granted. Don't smother that person (in other words, live your life). Give them no reason to ever want to stray. That's what you can control. If you trust your GF and know she loves you, and you follow those steps, you'll do fine.