Debora.
Every thing we do in our life past, present and future is controlled by things in our memory. So you can see that a person of fifty has more things to choose from than a child. The more of these things that are similar become core beliefs. Good or bad but none neutral. They have to be one way or the other to control our thoughts and actions. Short term thoughts that are based on these core beliefs are core thoughts. I won't go into the other types of thoughts in memory because they are not as controlling as these two. The strongest core beliefs are ones that have been built from early childhood and added to as we grow and age. Two identical children can have separate beliefs depending on how they were treated as children and later.
Twins that marry different types of spouses will have different core beliefs. If one child was Daddies favourite and another was Mommies they will have different beliefs. I'm telling you this so you know why you are different from your siblings if you are. It is not because there is anything wrong with you. In fact there is nothing technically wrong with you, you just have negative thoughts controlling your life. We all have different tolerances to core beliefs too so what controls one person might be ignored by another.
For most people core beliefs surround performance, looks and intelligence. But they can be anything.
At some time you have built a core belief that says you are going to get sick. If it has been around for a while it will be strong and you will believe it even if you don't want to. When you think about getting sick or you think about something that is going around you create a core thought based on the core belief that says you will get sick even if there is no basis or proof for this. This is not short term or present thinking that you can shrug off. This is core thoughts based on core belief influencing your short term present thought. The unfortunate thing is that short term present thinking if repeated reinforces the core belief. When these thoughts are not good they are called negative and the core belief is called a negative core belief. When they are good thoughts they are called positive. You can not remove either from your memory but you can decide which one will be on top. In other words more accessible and the one to control your thoughts and actions. Do you understand this? Any questions? I'm here to help with anything you don't understand.
Even if you do everything right the negative core belief is still available and the more times you use it the stronger it gets. This is why it is taking longer this time. And next time will be even longer unless you bury it so far that you have trouble getting to it and unless you build the skills needed to stop looking.
That is what CBT is for. CBT is changing unwanted thought (negative) for wanted. (positive)
That is changing thoughts of "I'm going to get sick" to more realistic to start with such as " there is no proof that I will" Or, "I might but it won't be that bad" Or better yet "I didn't last time so I'm just worrying for nothing"
CBT needs some help too. When the thought or the chance of getting sick passes you can't just move on to the next thing. You need to use that to reinforce or build if you don't already have one a positive core belief or you are still leaving the negative one to accessible.
So when the knee is better you have to keep going back to it and looking at it and saying, "I was wrong it is and was okay, I worried for nothing". You have to do this over and over. This counters the tendency to worry that you are getting from the negative core belief and builds a positive one to bury it.
Same with the mosquitoes. "It was a legit concern but I didn't get West Nile so I worried for nothing" Repeat it over and over. Repeat it when you start thinking about something else even.
I'm not saying you should drop all concern, I'm saying you need to stop it before it becomes worry.
Davit