Actually happy couples can have affairs! I read that in "Not Just Friends" and found it a little surprising. The problem isn't always happiness (in other words it's not a fool-proof method to avoid affairs) but boundaries and avoiding secrets... because sometimes A's can happen very gradually and innocently.
HMM2, I'm glad to hear you're on low level antidepressants. Just take vitamins, meditate, read positive books and let time and serotonin do it's job! Yes, you'll have ups and downs, but that's really normal.
I think there's probably a lot of people borderline OCD, or with OCD tendencies, and when things like this happen their reactions are just a little stronger. Time and those low dose meds should help. Hang in there!
There is no arriving, ever. It is all a continual becoming.