Let's try to look at just a few of these this evening.
First, there is a difference in nuance between "become" and "been." The act of becoming is different from the act of being. Let me give you some examples:
I have become tolerant.
I have been tolerant.
Do you see the difference in the two? In the first example, the person was not tolerant and then turned into a tolerant person, a change that extends into the present. In the second example, the person was in a state of tolerance that does not necessarily suggest this was a change for the person. There is also a nuance in the second example that suggests the person might not continue to be tolerant into the future, in spite of this being the present perfect. Picture a parent who is not pleased with his child's behavior saying, "I have been tolerant with you." There is a suggestion in that statement that the parent's tolerance is coming to an end.
Now, let us look at this interpretation that you offer:
I have become (gotten) less confident = I started to be less confident in the past and continue to be less and less confident into the present.
This is fine, with one exception. The degree of confidence does not necessarily keep diminishing as time goes on. Here is an example to help you see why:
I have become thinner as I have gotten older.
If my weight continued to decrease with age, I might disappear! But that is not necessarily what this sentence means. I was heavier, and I am thinner now, but I might not keep losing more and more weight as time goes on. My state of thinness continues into the present, certainly, but the degree of thinness is likely to have stabilized at some point. The same would be true of weight gain.
I want to go over one more thing with you this evening regading this sentence:
As I gained weight, I have become less confident.
In this sentence, your verbs are inconsistent because "gained" is in the past tense, and "have become" is in the present perfect. There are situations in which different tenses can be used because we want to show that something happened before something else, but in your example, words like "as" or "while" indicate a simultaneous action that requires the use of the same tense. Here is an example in which different verb tenses are used because I need to make a distinction between different times:
I had been thinking about changing professions, but I realized that the financial sacrifice would be too big.
In this example, I have used the past perfect tense to show that what I was thinking about in the past is completed and no longer exists, and as I continue, I use the simple past tense to show that my realization came after my thinking about a change.
English is a difficult language, but the fact that you have an awareness of the possible nuances and have a concern for precision will serve you very well.