You absolutely can be happy again, and you can be happy with your husband - Whether that's a mono relationship, a poly one, or whatever.
I love being a poly person. I have a great poly partner, and today went on a great date with a couple. And that's a luxury. It's not a need. Not dating multiple people will not tear your soul apart.
You keep talking about the attention, and maybe if you clarify to your husband what your needs are, for the attention, not necessarily the partner(s), you may find that sates you. Marriage comes first. I know some poly people disagree, but I personally believe in the power of one's word and commitment.
Being mono should not force you to be on antidepressants, and extramarital sexual behavior will not cure depression. It's no different than using alcohol, drugs, or food to deal with those feelings. Something else is amiss here , and before you make any life changes, you need to focus on your mental and relationship health.