Confused

So my 20th birthday is coming upin the next week and half I haven’t celebrated since I was 11 because that’s when my endometriosis symptoms began. Since then I’ve been terrified of going to parties, restaurants, parks, arcades. Pretty much I only leave my house to go to the store now a days. My boyfriend really wants to take me out but how do I explain to him that I have food anxiety and especially in public. I just got off steroids that made me gain weight because I was only 105 lbs for the past 5 years (now I’m 156). Does anyone have advice on an alternative celebration or a way I can explain my anxiety with out sounding like I’m relapsing into my disorder?