I feel like a different person

It feels as if I am a different person, with different capabilities depending upon what my mental state/thoughts are.
Sometimes I’m flowing with good thoughts and intentional models and almost in a dream land – that sometimes I worry is too idealistic and I won’t actually achieve.
Sometimes I feel as though I am in the pit of despair am about to crumble, something bad is going to happen.
They seem such different states. I sometimes feel like a fraud being the strong positive person, and sometimes feel embarrassed being the weak, despairing person – no one will want to be with that one.
I’m not exactly sure what to do about this.
I can say that I see how my thoughts create my feelings – I know that a feeling is just a vibration in my body, but that is easier to see now, when I’m not in the deepness of feeling so upset. When I am very upset, it’s rather overwhelming. I’ve sometimes been able to tell myself “this is just a feeling in my body” but this past week, being physically sick and emotionally challenged, I didn’t even remember to do that.
Any tips for this situation?
Thank you.