What are the ways to know if a person has genuine feelings for someone or they are faking it for some reason?

Faking is trying, it would need effort, you need to differentiate between the two, either is effortless or with effort, caring in tiniest of things would count.

I’m starting to feel unhappy with someone I love very much. He hasn’t done anything wrong, but I guess I just don’t feel it as much anymore. What should I do?

Perhaps, he doesn't have the qualities that you thought he had, or not up to the mark, or you deserve better, may what goes, but finally, you are over him, he has lost it, accept it.

How can I overcome this feeling of insecurity when I'm in love?

Why and from where this insecurity is arising? It is not natural in true love. Analyze deeply. And if actually, it is arising than it will show on your face and behavior which would affect your partner negatively.

Every time in my relationships, it feels perfect for the initial 3-4 weeks, but later on, I start to feel irritated with the person. His every action irritates me. What should I do? Is there any solution to this? Am I having some psychotic disorder?

Seeking sunlight without having the sun?? Where is that so-called “LOVE” ?? Without that genuine love, it would be natural to feel the way you are feeling. Don't worry you are normal.

My boyfriend keeps saying that he doesn't care how fat and unfit I am, I hate it because he keeps on mentioning it and I feel body-shamed. Am I over-reacting or is he intentionally making me feel ashamed of myself?

Somehow he is conscious of the fact related to your looks. Without flower you cannot have the fragrance, for example; a person who is not sad will never discuss happiness, it is just not required because to him happiness prevails naturally. He is being effected subconsciously or is aware of what he is constantly hammering