At 21, you are old enough to do as you please without your parent's permission.
The best that you can do is be firm but loving with them. Explain that you realize that you made a mistake marrying so young the first time, but that you have learned from it.
Sometimes when our children make mistakes, we over protect them in the hopes that they won't be hurt again.
They need to respect your decision to get married. They don't have to agree, but showing respect and support would be one less obstacle for you and your new husband to overcome.
My advice is to go ahead and marry your long time friend. Tell your parents that you understand their concerns, but are old enough to know what you want. Ask them for their blessings and the chance to prove that this marriage is what will make you happy.
If they still don't agree, go ahead with your plans and let them know that you love them and understand their concerns, but are doing what makes you happy.
Treat them with respect and when they see that you are happy, they will hopefully come around and agree that you made the right decision for you.