You have the right to say no, or to stop wanting sex at any time, and your partner should respect that – even if they want to keep going.
It also doesn’t matter how long you have been with someone or how many times you have done something; you can still say no. Just because you wanted something in the past does not mean you want it now.
Consider getting tested for STIs and encouraging your partner to do the same.
If your partner tries to guilt or manipulate you into having sex, this is abuse.