If you would like to reject a marriage proposal with a letter, here is a simple template you can use to write a letter for rejecting a marriage proposal.
Words can’t describe how happy I am that you would want to marry me. I am aware it is a new chapter in life. However, I feel like I am not yet ready for that stage. Thus, I’m going to have to respectfully reject your proposal. Marriage is something that can’t be taken for granted. For me, agreeing to marry someone is such a life-changing decision.
This is no knock on who you are though. I think you are an amazing person. This is just not the right time for me to get married. I hope nothing changes between us. Patience is needed for this situation as I’m confident we will get married when the time is right. Unfortunately, that time is not yet now.