It's been four months since graduation, you have most of your life figured out, and everything seems to be going smoothly. Except for the part where you just can't stop feeling sad, it seems like you're just not enough of a person, and you find yourself wanting to curl up into a ball and cry on the regular. First thing's first, it's okay to do that, but also start looking for ways to make yourself feel better. Here's some advice on what might work.
Let's start by manifesting the scenario in which you begin to feel this way. You had a long day at work, mining the potatoes. It was a fulfilling day and the foreman was pleased with your haul. You are walking home, feeling good about your work, when your twin, Jed comes around. Jed is like you in almost every way, except that Jed's an asshole and he just won't stop poking holes in everything you do. Here are some ways to cope with Jed.
Change your scenery. Jed is easily distracted by changes in immediate or familiar environments. Go outside, take a drive, hang a picture, paint the walls, rearrange the furniture, or just improve/turn on the lighting where you spend most of your time (something that has been scientifically proven to improve the mood of a majority of subjects).
Eat better, or (if you're already eating well) indulge yourself. A healthy diet will keep you and Jed feeling pretty good, and changing the diet will keep him distracted. Every now and then you can treat yourself and Jed to keep him shut up for a bit. Make sure you're getting the proper amount of vitamins and minerals, as well as an appropriate amount of calories from good sources (fiber, protein, certain carbohydrates). Make a healthy baseline and then supplement that with the unhealthy stuff you enjoy.
Do the things you love and strive to get better at them. Focusing on a hobby, movie, game, or book takes Jed's focus off of you and puts it onto something far more interesting. If it makes you laugh, all the better. Jed likes to laugh too, and will forget to berate you if he is amused.
Move around. Start working out, get those juices flowing. You don't have to do too much, even only 15 push ups a day is better than nothing. Work up to that, then surpass it. find a place to walk or jog. Get that heart rate up. Jed will feel too tired to dog on you after you both sweat for a bit.
If Jed starts to get too brash in what he's saying, the words start to hurt beyond the helpfulness of the above suggestions, talk to someone, preferably a professional therapist. Jed doesn't want to hurt you, he just doesn't know anything else to do, and a therapist can help you figure out ways to calm Jed down that an online advice columnist wouldn't think of. You are not weak for seeking help in dealing with Jed.
Finally, get angry. Some days Jed will be on your case and just ruin your day like he was born to do it (which he was, honestly). Don't give Jed that power. He's your brother but that doesn't mean you have to put up with his bullshit, and on those days don't smile and be the bigger person. If he gives you too much lip, smack that punk down. Put Jed in a headlock and make him feel pain. You are the king of the ring. When he gets on you about how few potatoes you mined compared to Hans, you get on Jed about how many he didn't mine at all, because he's a lazy fucker who just feeds his own ego off of undermining your success. And his face is stupid.