Hi, I'd like to help you with your question.
Your problem is not as uncommon as you think. Some people, particularly introverts, take time to express how they feel. They need extra time to think things through in order to come up with what they feel is the best solution. There is nothing wrong with this trait. It is good though, to be aware of it since you can then incorporate needing more time in making decisions and thinking things through when you are faced with a decision.
It may happen more often with your boyfriend since he is not as patient with your different approach to decisions. From your examples of his interactions with you, he sounds somewhat critical. This can put pressure on you and make you feel you are not able to make a good decision within his time frame. Telling you that the topic is over and done with and to drop it is a way to say that he feels your input is not as important as his. Even if you take time to decide, your input should be just as important. And telling you that you need to change is also imposing his view onto you without considering that you are an individual with your own way of thinking and making decisions.
Try talking with your boyfriend about putting off any non emergency decision until later. Let him know you need more time to consider all the information and find the best solution. To make it fair, give him a time frame such as one day, for example. This will help him know when he can expect your input.
Also, you may want to learn more about decision making and how it works. Developing a pattern to how you make decisions can help you make them faster. For example, if you are trying to decide how to spend $40, you can make a clear decision pattern for yourself by deciding what you will consider first. One example would be "do I need anything important right now?" and then move onto the next question you can ask yourself until you come to a decision.
Another way to help yourself is to write things down. Keep a notepad or other paper with you and write down significant points you feel you might forget. This helps you work on memory and retention.
As long as you are getting along in your life and do not have any trouble functioning, this problem is nothing to worry about. A little work on your part should help you overcome and adjust your decision making so you can feel more confident.
I hope this has helped you,
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