I've been road cycling since July 2017. I am an overweight guy, the cycling has helped shed some kilos but I'm not at my ideal weight yet, and most of my weight is stored in my belly. A month after getting my bike, I got a professional bike fit done, and it has made me very comfortable on my bike. However, when I go on group rides I see the more experienced riders always get into a TT position (forearms on handlebars) during long stretches of roads. I have tried to get into that position but its uncomfortable, and my peddling becomes extremely inefficient. My question, is this because i'm overweight and lack flexibility, and this will just come with time, or is my bike fit wrong?
A bike fit is not intended to make any position you can adopt on your bike work.
If the bike fitter did their job correctly, then would have set you up for your current level of flexibility, strength and desired riding style. You say you are 'very comfortable', presumably in the position you ride in most of the time, so the bike fit should be good.
If you have only been cycling regularly since the summer, starting from less than ideal physical condition, I would not be at all surprised that getting into a slammed position on the bike is going to feel pretty horrible.
The good news is that you can lose weight and improve strength and flexibility over time, and you will be find it easier to adopt a more aggressive position.
Regardless of your weight, holding an aerodynamic position isn't easy: it requires some core strength and flexibility. Even strong cyclists can have trouble with it. This video (jump to about 4:25) shows how much trouble a guy who can ride at 300 W (that's strong) can have holding a tuck for even a few minutes.
So what I'm saying is, don't stop working on it, but remember you've got lots of company.
I just went for a ride in the middle of the day. Normally I wear cycling shorts under my regular trousers/pants with a leather belt. However it was quite hot so I simply put the critical things in my jersey pockets and did without the pants.
The difference was significant - on the drops my thighs touch my belly every pedal rotation.
Yes it was a lot cooler for riding and I smashed a couple of PRs, but not sure if the inference between leg and stomach was less bad than the cooling was helpful.
So, try wearing a belt while riding, and see if it helps or hinders.