'My husband is just not interested in sex'

We have been married for two years now and haven't had sex in over six months. I have tried everything, but he simply isn't interested. I don't think this is because he doesn't love me anymore, because he clearly does and is very affectionate. I know he's not having an affair either, because he's simply not the type and I would have figured it out a long time ago if he were. Is there anything I can do to change this?
If you're sure he's still in love with you, it's probably a physical or psychological issue that's preventing him from being interested in sex. For some people, it's a hormonal imbalance, medical issue, or side effect of medication for a medical issue. For others, it's an emotional reason and, possibly, depression. Why he isn't interested is impossible
for anyone to diagnose without professional help. Has he considered meeting a doctor or counsellor? Have you asked him about it? I strongly suggest you convince him to speak to someone.

I intend to marry at the end of next year and was trying to figure out if there are stages that every marriage goes through? If there are such stages, maybe my partner and I will be better prepared.
If there were stages everyone could prepare for, I suppose divorce rates would be a lot lower than they currently are. I suppose what you can expect, however, is things moving predictably from a state of unabashed romance to a period of adjustment where the two of you understand and get to know each other better to, hopefully, a stage of rediscovering yourselves anew. These periods may be interspersed with the usual business of living that involves births and deaths, of course. The best part of marriage, according to people who have been together the longest, is when you know each other so well that you start completing each others' sentences. I hope you and your partner get to experience it.