Tips on hiding depression from my girlfriend?

I know the title seems weird but I'd like to quickly preface it. I've had clinical depression for the past couple of years, mainly because my parents have separated and I've had many family problems, and while I can normally hide my feelings sometimes I get into these deep lows, where I can tend to get very isolated/irritable. These "episodes" generally last a few days, however my girlfriend sometimes notices the way I'm acting and I have to try and find excuses. I can't let her know about it, since it'll make me seem weak and she'll leave me (this has happened with people I've been in relationships with before). I'd just like to find a way to effectively deal with this, so far meds and therapy haven't worked.
Tips on hiding depression from my girlfriend?
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