((((((Not Understood)))))))) Welcome to PC!
You are not alone in this and as Big Mama mentions there are not only prosthetics (breast forms) that can be inserted into the bra is looking to fill out your figure if this might help to feel more comfortable in clothes; there are also special bras that have prosthetic or padding built into to one cup to help. These can be used for the situation as you describe and are also marketed to those that have underdone mastectomies, if this helps in your search for them.

In being comfortable with someone, it matters more how comfortable you feel with yourself. Do what you need to to find your level of comfort and confidence, whether it be revealing, wearing a shirt, or lights low or off. The right person will love you for you no matter your concern; also especially considering we are our own worst judges and tend to more critical of our bodies than our partners tend may be. What is painfully obvious to us and seemingly a problem or issue, may not be to someone else.
I hope you find what works best for you.