Always there for my girlfriend but she's never there for me, what can I do?

My girlfriend and I have been together for some time now, I'm always there for her regardless of what it is she needs. When she's low I'm always there. Helping her constantly has got me feeling low and I'm reaching out for her to be there and she's not. She'll try to cheer me up by saying a joke or something, I'll smile and that'll be it for a few days. She told me that she can't deal with my roller coaster but not blaming her, just this roller coaster has been caused by the way she's been yet I don't have a heart to tell her out of fear I'll lose her.

ANYTHING will be appreciated, Thank you.
Always there for my girlfriend but she's never there for me, what can I do?
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