In order to change your thinking you need to know the exact thoughts and thought patterns that are troubling you or causing emotional distress. Some examples of counter-productive thoughts are, "I am stupid. I can't do anything right. I know I am going to fail. I know that person hates me. I hate that person." There are also specific types of thinking habits or patterns like catastrophizing, which means you often think the worst-case scenario will occur. Other thinking patterns include over-generalizing (thinking in always or never, such as, "I always mess things up."), mind-reading (thinking you know what someone else is thinking such as, "I know he hates me".), and predicting the future (thinking you know what will happen such as, "I am going to lose"). Make a list of the thoughts and thought patterns you want to change so that you can refer to them later. Write down which patterns or habits you tend to have. Be aware that some thoughts that may be considered "bad" are simply part of human nature, e.g. sexual thoughts, or "what if" scenarios that stem from our desire for personal safety. These thoughts are okay to have, and normal, and come from good instincts (like to procreate or protect ourselves and loved ones). If these thoughts become intrusive or interfere with your daily life, then you may need to address the obsessive nature of the thoughts. But remember, that still doesn't make the thoughts themselves bad. After you have identified your bad thoughts and thinking patterns, you can begin in develop alternative thoughts. Notice when you have an unhelpful thought. Identify the thought and then change it to something more realistic and appropriate. For example, if you thought, "I never do anything right," change this thought to something more accurate such as, "Sometimes I make mistakes and that's okay. I am human. I will do better next time." Catch yourself when you think a bad thought and say to yourself, "Wait a second! That is not a good thought and it's not true. I know I can turn this thinking around to something positive." A therapist who specializes in Cognitive Behavioral Therapy (CBT) may be able to help you with this process and give you additional techniques to try. (Note: CBT is not for everyone and for some psychological conditions a more nuanced approach is needed. Keep in mind that all therapists are human, and may not understand your condition. Try to find a therapist who admits that they do not have all the answers for everyone.) For example, if someone tells you not to slam the door, the first thing your mind does is imagine a door slamming. When you frame something with a negative grammatical construction "don't think about X" your brain has to think about that thing in order to remind itself not to. Subvert this by mentally replacing the undesired outcome with the desired outcome. For example, if you're worried about a job interview, don't think: "Don't forget my portfolio." Think: "Remember your portfolio." Instead of "Don't blow it," think, "I'm going to nail it." If you cannot stop worrying about how something could go wrong, flip the script and focus, instead, on the most positive possible outcome in that situation. Instead of trying to ignore the thing that is causing you stress, guide your thoughts in a direction that causes you less stress and anxiety. If your bad thoughts are about someone else ("I hate that person") ask yourself why you had that thought. Did this person do something bad to you? Is he or she abusive? Or is it something within you, like you're jealous of him or her? When you have these thoughts, examine your own emotions. What are you feeling right now? Do you feel insecure, powerless, or isolated? Try to figure out where these feelings come from. Did your parents always measure you against your perfect older sister? Try to shift your attention away from what others are doing and understand what is happening with you when you have those thoughts. Practice empathy by imagining sympathetic reasons for their behavior. Maybe you had a judgemental thought about your overweight friend, not realizing he's been busy caring for his sick grandmother and doesn't have time to exercise. Or maybe the person who was rude to you is suffering from chronic pain and lashing out because she's hurting. It doesn't need to be an accurate reason, but it might be enough to keep you calm and allow you to move on.
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One-sentence summary -- Identify the thoughts you want to change. Change your bad thoughts. Substitute positive wording and imagery for negative ones. Consider the best-case scenario. Address bad thoughts about other people.

The main reason for going to a dermatologist is that they can tailor products and skincare routines specifically to you and your concerns. Research in to potential dermatologists you want to see. Read reviews and articles on the company and actual doctors to see if they are legitimate, and suited for your needs. Professional help on stubborn skin is help you may not be equipped to provide on your own. Consider seeing a dermatologist as a last resort. Try using home remedies and changing your diet for at least two months to see if your skin clears or improves in ways you want it to. If this fails, then find help. Scar removal can be a priority for those who want healthier looking skin. These procedures can be expensive so make sure to look in to local professionals who can administer them at a price reasonable for you. This is a quick fix for visibly different looking skin. It can be done on your own with at-home with bleaching creams, or scrubbing brushes. Look for microdermabrasion or dermabrasion procedures to remove top layers of damaged skin. Removing scars can also even out the pigmentation of your skin. If you suffer from stubborn acne or other issues, a dermatologist can either recommend the perfect product(s) or procedures to get your skin back to its healthy looking self. A dermatologist can help you understand your own skin, why it is the way it is, and how you are perpetuating the problem or starting new ones. Even if you don't have a chronic skin issue and you notice something isn't quite right about your skin, it doesn't hurt to go in for a consultation with a dermatologist just to see what's happening.
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One-sentence summary --
Seek out a well-established and knowledgeable dermatologist. Talk to a professional about removing (acne) scars. Stop chronic skin issues so you are no longer bothered.