I don't want to marry you

It is so easy to get caught up in the excitement when someone proposes to you. If you really feel that it's best you not marry someone then honesty becomes your very best option.

There must be some reason for your decision, and I'd try to explain those those reasons as best as possible. There is most likely no way you'll salvage the relationship, but you can break it off with as little pain as possible.

Whatever decision you make in how to tell him make sure you do it privately and without a lot of voice raising, yelling or other unusual style. Do not do it in front of friends or family members, that will cause even more hurt and pain.

Take it from one who knows, if you were to marry him in spite of feeling it wasn't the right decision you'd live to regret it (and unfortunately so would he).