Seasonal depression and sex

So I’ve been with my boyfriend for almost five years. I’ve been dealing with the worst depression that I’ve ever had to deal with. My boyfriend is great, he deals with it from time to time too so he can relate and he’s always here to give me advice and let me vent. I love him. That’s not the problem. I don’t know why but I feel so disinterested when it comes down to us having sex and I don’t know how to change that. It’s like when it comes time for us to sleep and he wants to have sex I can’t get turned on at all. I’m just numb. I don’t know what to do or how to change this because I’ve never had to go through this before. Any advice is appreciated.