Real and fake happiness – does such a thing even exist?

Greetings to all answerers, readers, ignorants and questioners 🌆

So the following:

(If this is important for the answer, then read from here. If not, scroll all the way down)

• I have been in an episodic depressive low for about 4 years and can't get out
• I have a lot of friends but no one who is really seriously interested in me. The ones who are even slightly interested don't have time.
• I have a pathological indifference, am dissatisfied with my figure and, no matter what I try, can't do anything about it. I feel somehow completely alone in this world (family excluded) and, despite regularly going to the gym, I have physical pain all over like a hard-working 60-year-old, combined with tiredness no matter how much sleep I get, which leads to a lack of motivation that makes me happy if I manage to change after work.
• Blah blah blah. Who cares.

Question: Suppose I'm in therapy or actively doing something about my years-long struggle with mental health issues, and I'm "happy" afterward. How do I know that this feeling of happiness is real and that it's still the same feeling I had before all my problems? Maybe I'm just imagining that this feeling at this point in time is "happy," even though it isn't. In that case, I'd just be living with a lie or in an eternal illusion. Maybe my mind wants me to believe this so badly that it doesn't allow any other thoughts. But for me, that would be too far removed from my personality (if it's even still the same) and thus from my "free will." Go ahead, or don't go ahead.