I’m so scared😔

Are feelings suppose to change in a long term relationship? Not feelings really but the relationship itself? I’ve been with this boy for 4 years and he is my everything😢 I can’t see myself without him atleast not rn. He’s my bestfriend not just my bf he makes me so happy.., I’m so scared I’m falling out of love with him I don’t want to at all.. I just don’t get the butterfly’s really anymore, I don’t get super super excited, I’m literally so terrified.. when I think of leaving him I just want to cry.t feelings now are more like contentment I’m super comfortable with him I could literally tell him anything and everything, even when we don’t do anything and just sit and lay there together it’s not awkward or anything I want to be with this boy, I’m over analyzing ever little feeling I have it’s so exhausting, I was just recently diagnosed with GAD idk if that has to do with anything this has been going on for like a month these stupid thoughts of falling out of love and I’m so frustrated. I’ll have moments of clarity where Ik it’s just my thoughts but then I’ll over analyze a feeling and I’ll start obsessing over it. This can’t be happening to me... I’m so terrified 😔 has this happened to anyone? Is it normal? Does it mean I’m not in love anymore and have to leave?