'Our badly behaved son is breaking our family'

My wife and I constantly fight about our son, or no apparent reason. He's badly behaved, and we keep blaming each other about this. I'm afraid this marriage will end at some point if we continue like this. Is there something we can do to stop fighting?
Parenting is a process, and I really can't tell you or your wife how to raise your child. What I do know is that children always change the dynamics of any relationship, which means a couple always has to work at getting things back on an even keel. If you are worried about your son's behaviour, you should both consider speaking to a counsellor who can try and figure out why he behaves the way he does. Your fighting may inadvertently make things harder for your son, making this a vicious cycle. Sit down and talk about why you both seem to be fighting a lot more, and for no apparent reason. Neither of you has a grasp on what the root of the problem is, which is why speaking to an expert is strongly recommended.

A lot of people in relationships talk about how trust is important, but I don't know what they mean. My girlfriend and I have been together for a year now, but I'm not sure if we trust each other as much as we did when we first got together. Are we doing something wrong? Is our relationship okay?
I think you're overthinking this. Just because people believe your relationship needs to be defined according to rules set by them doesn't mean your girlfriend and you have to follow them. Do what makes you both comfortable. Every relationship is different. If you are both perfectly okay with the way things are, stick to it, spend more time together, and assume that you will end up trusting each other more anyway. To know someone better, to understand and accept their strengths and weaknesses is all part of the trust-building process. It isn't really rocket science.