I commend you for wanting to start so late on. Suffice to say, you need to get used to the hits and the contact especially if you intend to play competitively. I've met many people who take up rugby relatively late on, get properly hit and just sack it off and never play again. For your experience and size, I'd suggest trying as a blind side flanker (in the pack but away from the direction play when in scrums). Its a relatively simple position in that your job is to support your ball carrier and tackle the opposition when they have it. Support and tackle, thats all you do. Some of the best flankers of all time (Neil Back, Phil Waugh) were smaller guys, though the game has come a long way since them. The hits though, they can be a real shock to the system. Don't dip out of the contact sessions in training, hit tackle bags, run into people with the ball. Go in 100% because otherwise you'll end up hurting yourself! It is certainly a mindset. Just ask your training buddies for help too. Doesn't matter how fit you are. If you can't take the physicality of the game, you'll fail.