Does this sound like SAD?

Hi everyone, im just made an account on this site because I was really curious about peoples opinions.
Im just 17 right now(12th grade), ever since 9th grade every winter I get really depressed and self conscious. I doubt myself and do not have any energy at all. All the time I worry about what people might think of me and if im doing something wrong. I am constantly having issues with my stomach and could never figure it out until I read that can be a symptom. I have a bf of 3 years, kind of. We always end up breaking up around January because he just gets fed up about how depressed I am and that I never want to go out, then get back together around April when my mood lightens. It was actually him that thought of this a couple days ago. I start avoiding people just because I always want to be alone and don't think they would want to talk to me anyway. In the summer I get my life back together and feel great, then fall comes around and theres a huge spiral downward. I used to have an eating disorder, which seems to creep into my life during this period, and I also cut sometimes too because of the feelings of worthlessness. Although im usually miserable and out of it, I have some good days during this period once in awhile too though, which is why im not sure.Also I can not concentrate on anything! My mom doesnt think theres anything wrong, but I do. How can I tell her without her just shutting me down?

Thank you so much Bagpuss, im going to try getting one of those lights, do those work? And going to the doctor is definitly a good idea. Thank you!

Awe well good:D im glad to hear that! Im going to try and go out this weekend to get one. Thank you so much again:-]