Hi, im 18 and a senior in high school. I used to be in good shape, football, sports, really athletic, then my doctor put me on medicine for anxiety. I gained a lot of weght. I am now 279lbs and 6' 1". I got off the medicine and have lost. Even as far back as i can remember i was very insecure about my chest area. I wear tight shirts underneath to hide it or wear sweat shirts. Ive found a girl that i thhnk likes me but with dating im really insecure about having sex due to that. Is it a huge factor and turnoff?

Don't worry about it. It's not a big deal.
Matter of fact, I would make it a point, after getting to know her, to tell her how you are feeling about your body, that way it is off the table and is in the past.
Don't dwell on it, and move on.
This is not a big deal, trust me on this.
I wish you well my friend.
**As a note. I'm glad you have lost weight, as it is not healthy to be overweight. If you still have concerns about your weight, see a sports medicine physician, and follow his advice on diet and caloric requirements.
Bye.
Thank fully i despise SERIOUSLY OBESE people , so i have never even gotten near the man boob shape