I dont know what to do

I have this friend who I have been friends with for about four years. In the last year she has changed she is no longer optimistic but rather constantly depressed and I listened to her and try to be their for her but its becoming a burden because she doesn't get my advice and she never really wants to talk or hangout unless its about her being depressed for being alone and not being married at age 30 or whatever, I am 30 too but im not constantly bringing ppl down and try to be optimistic. Should I let her go or continue to support her??? I just feel like are relationship has been one sided for a while and try to tell her but she continues to do same thing.