It seems to me that there are two different approaches to answering your question. The first is to focus on what the structure of a good footnote is, what information is suitable for conveying in a footnote versus the main text and what an appropriate length is for a footnote. Taking that approach, is, I think, unlikely to be useful to you. The fact that you can recognize well written, appropriate-length footnotes in the writing of other people suggests that the problem for you lies elsewhere. Hence the second approach to your problem ...
Your experience, in terms of your internal reaction to your own work, sounds very similar to the experience of people who more obviously have what is referred to in the psychology literature as "performance anxiety". Although performance anxiety is usually associated with such things as being on stage (hence the related term, "stage-fright"), its critical features have little to do with the specific situation. Rather, performance anxiety usually includes:
- Negative appraisal of one's own performance
- Anticipation that one's performance will result in negative appraisal from others
- Concern that the negative appraisal from others will, in some way, have very bad consequences.
Your description of the feelings and concerns that you have as you elaborate your footnotes in ever greater detail suggests that your are trying to forestall the possibility of points 2 and 3 above. My reason for introducing the term "performance anxiety" is partly because it helps to structure an answer to your question, but also because you might find it useful to look for literature on the subject using those words.
Now to a how you might approach the problem ...
It is generally very difficult to deal with anxiety in the absence of accurate information about the outcome of a behaviour. In your case, you might have tried to write shorter footnotes but found it almost impossible when your whole being screams out that this will only lead to disaster (e.g., other people being critical of you). My guess is that you have tried to do too much, too quickly.
With those preparatory thoughts, I suggest two things ...
- Find an editor you can trust, not only with your work but with your feelings and concerns about your work. By editor, I don't mean a professional editor; rather, I mean someone (perhaps a colleague or friend) whose judgement you trust and who (a) would be willing to work with you in overcoming the problem, and (b) can help you to tailor the footnotes in a way that fulfulls the requirements of the next ingredient in this two part list ...
- Together with your editor friend, take a graded approach to overcoming your problem.
- Begin by trying to trim down the length of the footnotes in something that you are not deeply invested in, either emotionally or intellectually; for example, look at where you have, in the past, written short footnotes (perhaps in class lecture notes) and see whether you can eliminate the footnotes entirely, or shorten them even more. You should be aiming for a length of footnote that leaves you with a slight tingle of apprehension about whether it's OK or not, but low enough apprehension that you can move ahead with your plan. After a while, I expect that you will not feel much apprehension about this at all.
- Having experimented with a few low-stakes situations over a period of a few weeks, you should feel able (again with your editor) to experiment with shortening the footnotes in some pieces of work that are more serious. Ideally, if step 3 went well, then the apprehension you feel at this step will be no greater than it was initially for the previously situation. Once again, your aim should be to produce work where you feel some slight discomfiture about the shortness of the footnotes, but certainly not any feelings of dread.
My guess would be that within a few iterations, you'll be feeling a lot less anxious, and your work will more closely approximate that of the people whose work you admire.