explaining depression to my SO..

hello! so, i'm not diagnosed as clinically depressed- as in i do not take any depression specific medication etc.- but my dad has a long history with very serious depression and i've had mild depression since middle school. since starting high school i've been more aware that what i was feeling was depression, and since my grandmothers death last year i am just a lot more sad at odd, random times where i want to cry or be quiet without necessarily being alone. 
i've gotten my first, amazing boyfriend about 2 months ago. we were always very close and i disclosed to him once that i had was very sad in middle school, but don't know if he remembers. but, the past few weeks i've been super on and off sad, and we didn't really do that much for valentine's day (at my request) so he thought my sadness on the phone and in general today was because of that. 
i don't really share my feelings with anyone. even with the psychiatrist i go to once a month- it simply makes me feel uncomfortable, which i know isn't healthy by just how i cope. my boyfriend is very much about openness and communication, and i don't know if he gets how hard my family/home life is for me mentally, and that this sadness is depression. 
has anyone had to tell a SO about depression, anxiety, anything similar? mine definitely isn't very familiar with any of it, but i'm confident he would be understanding and supportive. just since i don't share feelings much, how did you share this personal thing? 
i know this is long so thank you for reading and sharing if you do!!! xx