All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Indeed you did well by regretting this forbidden and evil habit of masturbation and you should sincerely repent to Allaah. A sincere repentance should fulfill some conditions so that it could be accepted; in this regard you may refer to Fatwa 87903.
On the other hand, you should not worry about your premature ejaculation or consider this as an obstacle to marriage. Premature ejaculation could be just an illusion and not real, and when someone marries he will discover that the fact is otherwise. It may also be due to staying single for a long time, because when marital relationship with the wife lasts a long time, then this problem would be solved or at least reduced.
If we presume that this matter continues after marriage, then it has a treatment, Allaah willing, as Allaah has put a cure for every disease. Jaabir narrated that the Prophet said: “For each disease there is a cure, and when the (right) treatment is given, the disease is cured by the Will of Allaah.” [Muslim] So, you may consult specialist doctors who would guide you to a treatment that is not forbidden contrarily to the impermissible method which you mentioned in the question.
Moreover, we advise you to mention Allaah and seek His Forgiveness much as this helps in facing hardship and facilitating difficult matters.
As regards the device that you mentioned, it is not permissible for you to use it even after marriage, let alone while you are still single as it is not permissible for a Muslim to do such an act that contradicts the saying of Allaah (which means): {And they who guard their private parts. Except from their wives or those their right hands possess, for indeed, they will not be blamed.}[Quran 23:5-6]