Airmen25,
Welcome to the forum, and indeed the industry. As has been pointed out, if you can cut through the BS on here, there are some really great guys contributing on here, and it's always a good place to come for constructive conversation and letting off steam.
Myself, I am currently operating the DH8, but I have spent 2 years on Embraer RJ's.
One great piece of advice I was given has stood me well. As a cadet of a major European charter company, I was told in the early days that they wanted more confident, assertive First Officers on the line. They were finding new FO's being too docile, and not standing up to Captains even when they themselves were blatantly wrong.
So how does this translate to the real world? I guess, for you right now; hit the FCOM and your Operations Manuals. Absorb all that you can and leverage your other company crews for advice, "what would you do if..." etc. Armed with job knowledge and a little experience, your confidence will grow naturally.
Share your mental model. Possibly (my opinion, possibly wrong) your hesitation meant the Captain felt he had to do something. When he suggested using Speedbrake too close to VLS, that was the opportunity to explain you were not comfortable doing that. Always let the other guy know what your thinking, or planning.
Don't over complicate matters either. Energy management, especially in a slippery jet is an art form. Getting high and fast is an issue and will happen to you again. You can recover slight deviation by either killing the speed, or the height, and some line experience will tell you how much is possible.
In your example, would an extra 5 miles have solved the problem? 5 miles is what, 2 mins airborne? Surely the better outcome than forcing the matter and having to go around because your above the profile. There is no shame in this!
Lastly, debrief! A quick chat with the other guys, even Capt. Frontbottom, with a "what would you have done in that situation" could result in some interesting answers. In the early days, take notes of these Golden Nuggets, or keep a training diary and you'll find yourself up to speed in no time.
The early days can go past in a blur, don't get hung up on one guy being unfairly harsh, and try and enjoy it!