Just because you are in a relationship it doesn’t mean you have to have sex. It should always be your choice as to whether you have sex or not, even if you are in a relationship.
And just because you may have done it before it does not mean that you have to do it again, even if it is with the same person.
From the age of 16 you can legally consent to sex (this includes sexual intercourse, penetrative sex and any sort of sexual touching) but 16 may not necessarily be the right age for you to start having sex. Many people do not have sex for the first time until they are older.
There are no rules about how long you have to be going out with someone before you have sex with them. Being ready happens at different times for different people – never decide to have sex just because you feel pressured.
You're the only one who can, and should, decide whether you are ready or not.