Not being in the mood?

Okay so me and my partner have been having sex for about 11 months, but recently my sex drive is just really mute? I'll kiss etc, but actually having sex isn't really something I actively want to do very often anymore 😩 it's stressing me out because I have to say before anything happens "I'm not in the mood tonight" etc, and I feel as though I have to make sure he doesn't get too into making out because I don't want to 'let him down' so to say, even though I know sex is a two way thing, but he gets excited about it so easily which is flattering but I'm just really not up for it much lately and I hate leading him on, so to say.