Self confidence issue

I’ve been with my partner for 13 months, at first sex was amazing and mind blowing all the time but lately I’ve been feeling very self conscious of myself and when he tries to get me turned on I go inward and start worrying about myself and how I look. He has never given me any reason to think bad of myself he’s supportive and reassuring I have talked to him about before I just dunno how to get my head back into the moment and stop worrying.