All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Before answering your question, we would like to draw your attention to the religious dangers that you could have faced by linking yourself with this man in an illegitimate relationship for this long period. In fact, if the relationship you have had with this man involved any forbidden matters, like staying alone with him, talking to him, letting him look at you or even going into more intimate relationship with him, then such a relationship is prohibited in Islam. You should repent for it, ask Allah to forgive you and do as many good deeds as you can. We hope Allah will forgive you.
As for the answer to your question, if this gentleman is pious and trustworthy and if you are convinced that he would be a good husband for you, then you should try to convince your parents of this, reminding them of the prophet's Hadeeth which reads: “If a man whose religion and moral character are pleasing proposes to marry your daughter, then marry her off to him. If you do not do so there will be corruption and great evil on the earth. They (Companions) asked “If there is something (should we accept him)? He replied: “If a man whose religion and moral character are pleasing proposes to marry your daughter, then marry her off to him.” (He repeated it thrice.) [At-Tirmithi]
Convince your parents that their reservations in this matter are not accepted in the Sharee’ah. Being young is not a disadvantage if the young man is intelligent and mature. Being Handsome or cute are not the most important characteristics of a man. It is even said that a man is known by his perfection and not his beauty. You could also tell them that your happiness in the future will not be achieved unless you marry a good man who is appropriate for you from your own point of view. Such conditions are present in this young man. This man should make his self acceptable to your parents.
Allah knows best.