Sometimes we're so sad that we don't even want to feel happy. I understand that. I've been there.
The fact that you are looking for a way to feel happy, though, is a great sign! It means you are over the hump and ready to feel happiness again. At least you're looking into it, anyway. :)
The obvious answer to feeling happy is to do things that make you happy. But you probably aren't really in that space right now. You probably are tired of feeling sad but don't have much motivation to do anything that optimistic.
There's always the first step, however. And a great first step is writing in a journal.
Journal Topics When You're Sad:
The reason(s) why you're sad.
Things you are grateful for (they're there... I know they are!)
Do a brain dump - write non-stop for 30 minutes about anything and everything that enters your mind. (It feels great, and if you do it a couple of times, you'll start to work out of the hole you're in!)
Write a letter to yourself as a person you admire. (If you admire Oprah, what would she tell you in a letter?)
You also could turn on some good music and dance! I know. That takes a lot of energy. But it's a 3 to 4-minute commitment. You can do that! And you'll feel better after you finish. You might even take advantage of the endorphins and follow up that dance with a special bath for yourself or maybe tidy up a bit.
When I have been this down (and depressed), I would have to force myself to do something, and if it felt good, I would ride that wave as long as I could. It seems to increase in how long it feels good, the more I would make myself do something. It always gets better. Always.
This is a good add-on to the previous suggestions or a good place to start if even those ideas were too much. Visualize yourself happy. Don't let your negative thoughts get in the way! As you're going to sleep, just see yourself when you're happy - past or future.