Because it's a cover letter, I think it's quite natural that you say a lot about you in it. That's actually not to be avoided, because you want to give the addressee a good idea of your background, your motivations, your interests, etc. In short, you want to show them who you are, so they want to work with you. This is a totally different exercise than usual academic writing.
Now, regarding the redundancy of I, it is a matter of writing style. It probably wouldn't bother me much, but if you want to diminish it for some reasons, here are worthy alternatives:
Use constructions that, while retaining the first person, shift from the subject pronoun to other cases:
It has been my intention for a few years now to shift my research interests from pure psychology to experimental psychohistory, and I have thus taken in 2009 a post-doc position at the University of Trentor (group of prof. Seldon)
instead of “I decided a few years ago to move to the field of psychohistory…”. Similarly, you could say
Be aware that there are downsides, though: most of these alternatives are longer than a direct sentence starting with I, which means overusing them could make you sound windy.