Your feelings are understandable; from time to time I get letters that are similar to yours. But your mother's situation is also understandable, and the best thing you both can do is to try to understand each other and avoid hurting your future relationship.
Have you tried to analyze why you feel the way you do? I don't know the answer, of course; perhaps you feel like your mother is betraying your father by planning to remarry. Or you may worry that she is going to expect you to have the same kind of love for her new husband as you had for your father and you know that isn't possible.
But whatever your reasons, ask God to help you look at this situation through your mother's eyes, and not just your own. In other words, ask God to help you put her happiness first, and not just yours. Loneliness can be a heavy burden, especially after the death of a beloved spouse. The Bible says, "Each of you should look not only to your own interests, but also to the interests of others" (Philippians 2:4).
Above all, don't let resentment or anger eat away at your soul. They will not only harm you, but they will hurt your relationship with your mother. Instead, the Bible says, "Rejoice with those who rejoice" (Romans 12:15).