Every time you think badly of yourself, brood over past failures, or rehearse for negative outcomes, you risk programming your unconscious mind to believe that those things are what you actually want, rather than what you most fear. And as your unconscious mind always works very hard to give you what you want, the result can be (and often is!) damaged confidence, poor performance, unecessary stress, and the kind of negative emotions that can lead to physical and behavioural problems.
It's really important to appreciate that imagined success can equal actual success, deliberately rehearse for positive outcomes, and try to think responsibly and positively all the time.
Thinking responsibly and positively permanently means paying close attention to what your concious mind is doing, because most of us are, without being aware of it, involved in an on-going internal dialogue, a sort of subterranean grumbling mutter that only becomes obvious when we pay attention to it - and it merits close attention because negative thoughts tend either to recur and follow a familiar, brooding, pattern, or to hurtle in and out of consciousness very quickly. In either case, we often fail to grasp their significance, or appreciate that they might be incorrect, illogical, irrational, distorted, or potentially harmful.
Making a deliberate effort to monitor your internal dialogue by listening to what's going on in your own head will enable you to challenge your negative thoughts thoughts - subject them to rigorous, rational scrutiny - and begin to understand them. Then you can either plan to change their causitive factors, or dispose of them altogether as being completely false or inapplicable to your present circumstances.
It's quite useful, incidentally, to keep a diary of the things you catch yourself thinking about, because once you've got a couple of weeks worth of 'negative nuggets', you'll likely find that you are able to spot recurrent statements are that are self-damning or self-judgmental (or thoughts that are irrational, illogical, or inconsistent with reality), and will be able to evaluate their potential effect on your life, ask yourself questions about them, and make plans to deal with them.
Good throw-away-technique: call your diary "Things I Don't Need Any More", and make a ritual out of burning the pages every couple of weeks or so. Tear out the pages bearing your negative thoughts, drop them into the fire, and say: "And as I drop all these useless damaging thoughts, all my bad feelings about the past, one by one into the fire, I can look forward to good times coming as they are destroyed and transformed into smoke and ashes".
