I think if I could give any advise to a newly returned missionary it would be this: find something that you can put your whole heart and soul into, and do it as soon as possible.
As a missionary, one is busy all the time. We are trained to fill every hour with a meaningful activity, and it is things that matter.
When you get home that is one of the first differences you will notice, and one of the biggest adjustments.
So along with all the day-to-day things you will do, find things that matter to you. Set goals and set out to achieve them. Immerse yourself in furthering your education, building a career, continuing to share the Gospel of Jesus Christ, starting a family, anything that is good and keeps you anxiously engaged. Your biggest enemy to staying happy is to not have a purpose. Your biggest ally is to find things that matter to you, ways you can still help others, and do them.