Do not Give up. The reasons you regained weight may not be as simple as the overly simplistic judgments of yourself that come to your mind ("I have no control and I ate too much or I'm lazy and I didn't exercise enough.") Instead of letting your frustrations get the best of you, aim to understand the underlying reasons that contributed to weight gain. Chances are, it's much more complex (difficulty coping with stress, relationship issues, depression/sad feelings that leave you feeling lethargic, etc.) Until you understand what is underneath your behaviors and address those issues, it is very difficult to maintain a change, even if you know what to eat and how much to exercise.
Do not beat yourself up. You have regained some or all of your weight and of course you're disappointed, but saying cruel things to yourself will only make it worse. In fact, try pretending one of your closest friends was in your situation; feeling down and like a failure because they regained their lost weight. Write down or think about what you might say to them. Work towards trying to be as supportive of yourself as you would be with one of your friends.
Do not over-exercise or drastically restrict calories. If you've decided to try to lose weight again, it's tempting to not eat enough and over-do it on the exercise in order to see those immediate results. This will most likely work against you and could lead to more yo-yo dieting and further disappointment. Instead, try setting small goals that are achievable. If you are not working out at all, aim for once a week. If you are drinking a Grande latte 3 times a week, try making it a tall, 2 times a week.
Do not be too embarrassed to seek support. You are probably harder on yourself than any family member, friend, trainer, doctor, or dietitian ever could be. We are not here to judge you; we are here to give you support. Take advantage of the complimentary check-ins with your 20/20 doctor or simply seek support in the form of a workout partner. If you feel like you know what to do, but are having a hard time actually doing it, it may be helpful to make an appointment with a counselor to explore what might be getting in the way.