Is it obligatory to repeat the prayers that were performed with a lisp in the letter 'R' while being unaware of it, and is it obligatory to learn the correct pronunciation of the letter?

Your previous prayers are valid and do not need to be repeated, because you did what was within your capacity. If you can now pronounce the letter 'R' almost correctly, your prayers are also valid. If you pronounce the letter 'R' from its known articulation point, that is sufficient for you. However, if you pronounce it incorrectly, such that it sounds like something other than 'R', and you are capable of learning the correct pronunciation, then strive to do so to the best of your ability. You must also ignore obsessive thoughts and pay no attention to them.