How do you improve or get rid of your mistake of adding or deleting medd al asli
This mistake needs two important things, the first that the student listens to recordings of known masters of Qur'an recitation and repeats after them. The student should look carefully at where there is a natural medd and where there isn't and try their best to lengthen the two vowel counts when it is needed, and not to lengthen when there is no medd. There are many websites in which an aayah is repeated up to 7 times which one can benefit from. One of which is:
The second very important step, that the student recite to and be corrected by a qualified teacher. It helps if the teacher can imitate the mistake and then read it correctly so the student hears the incorrect and correct way of reciting the words or phrases which are difficult for him/her.
If you put in a lot of time listening and reading and then being corrected, insh'a Allah this will be corrected quickly.
May Allah make it easy for you and all to recite the Qur'an correctly.