Anxiety, Grief and Depression can alter our emotions and our decision making ability. Negative emotions make us see things differently...we see fewer options that are available to us ...we think in terms of "all or nothing"...we think the worst case scenarios in the situations we come up against... we tend to think things will never get better, etc... these things are called cognitive distortions.
There are some people who have dealt with negative thinking their whole lives...they see the bad before the good, they see all that is wrong, instead of the things that are right...they don't know how to live with an attitude of gratitude or to look for the positives in a situation. Not every situation is good...but God is always good. He promises to bring good out of the bad for those who believe in Him. We have to learn to live with our eyes on Christ not our circumstances and to live in expectation that God will fulfill His promises in our lives..
If you've ever tried to "just stop thinking" about something...you know that it doesn't work...it boomerangs back to you and you end up fixating on the thought even more. We have to replace negative thoughts with better ones...that is the way to overcome negative or self-defeating self-talk. We have to learn to be intentional about the things that we think and the thoughts that we entertain.
The Bible says in Philippians 4:8-9 "Finally, brethren, whatsoever things are true, whatsoever things are honest, whatsoever things are just, whatsoever things are pure, whatsoever things are lovely, whatsoever things are of good report; if there be any virtue, and if there be any praise, think on these things. Those things, which ye have both learned, and received, and heard, and seen in me, do: and the God of peace shall be with you."
2 Cor 10:5 tells us to "take every thought captive".
Romans 12:2 TLB says, "Don't copy the behavior and customs of this world, but let God transform you into a new person by changing the way you think. Then you will learn to know God's will for you, which is good and pleasing and perfect."
We can't just let negative thoughts go unchecked in our head...Max Lucado says "we can't always stop negative, fearful or anxious thoughts from coming to our door, but we can choose which ones we will entertain." Here are some great articles and videos that will give you tips on replacing and overcoming negative thoughts.
It's also helpful if you have negative tapes in your head that you play over and over again...if you have negative, hurtful words and comments that people have said to you that have left scars and skewed your perceptions of yourself, you need to erase those tapes and feed your mind with God's word...you need to let the things that God says have more power in your life...you need to focus on how God views you and perceives you. He loves you enough that He sent His Son to die for you. He has known you before you were in your Mother's womb. He says that you are fearfully and wonderfully made. He calls you His heir and child. Many of us have scars and wounds from our past, but we do not have to let those things define us. We can't always control the things that happen to us, but we can control how we will react to them.
We can learn to retrain our thinking and shut down anxious thoughts, negative thoughts, self-defeating thoughts, fearful thoughts. If you deal with OCD and you get stuck in your thoughts, it might be harder for you than for the average person, but there are things like Cognitive Behavioral Therapy that can help in addition to prayer, meditating on Scripture and focusing on Christ. Christ can help us deal with these difficult areas...