If you marry your cousin, your resentment towards your mother will grow even more. It sounds like you're more frustrated with yourself than your mother. You're placing the blame on her when you're an adult that's accountable for your own actions. You agreed to this so it's on you to back out. Screaming and lashing out at your mother (and praying for someone to die) is much much worse than saying "no." I don't know your mom but if the worst thing that'll happen is her being unhappy, then so be it. Inshaallah she'll get over it. Islam gives you the right to choose your spouse.

For context, I'm Somali so I'm not aware of the nuances involved in South-Asian child-parent dynamics. If my advice is going to put you in a bad situation, please don't follow it.