1. Communication
2. Finances
3. Sex
Not necessarily in that order, but these are the three most important parts of marriage.
When one is out of whack, 1/3 of you marriage is down.
When two are suffering, 2/3 of you marriage needs help.
If all three are sad, start working on the communication part first.
Make sex a quick second.
Then work on your finances and live with in your means.