Don't think of it as rude. Think of it as a blessing.
There are a lot of websites about letters of rec and the different types. I referenced those alongside a checklist of what LOR's should include (a handout provided by my undergraduate advising office). I know this may be difficult, but don't hesitate to compose a very flattering LOR for yourself. This is because your advisor will take several sentences or even paragraphs and just cut and paste what you've written. As a result, you want to present yourself in the best way you can. However, be realistic. Don't say that you're the next Mother Theresa or anything.
I had this experience w/ a G.Chem prof that I had in my freshman year. I only had him for the quarter, came to office hours, and checked in w/ him about once a quarter. Personally, on a scale of 1-10 of how well I knew him, I'd give it a 6. But, when I read the letter he'd written for me, it was REALLY good! Nearly the entire letter was identical to the one I had written...the structuring was the same, but actual words were reworded. To this day, if I need to feel better about myself when I'm blue, I just bust out the letter and read it.
I hope this helps. Good luck. Composing your own LOR just helps out your writer and saves them some time. It is difficult for them to know what adcoms are really looking for, especially if they have had little experience writing letters.