It's rather annoying because up till now, I was able to win sparring against other by just applying pressure until they got tired and I won.
Welcome to BJJ! Against people where you are at more or less at the same technical level, factors like athleticism, strength, size and aggression are usually what seperates people in a fight. Roger Gracie is probably the best grappler in the world today. He's beaten pretty much every top BJJ practioner in the world. He is great technically, but what helps get him to the top of the pile is that at 6'4" and 210 lbs he is usually a lot bigger than his opponents.
You have to, as the Chinese say, "eat bitter" when you start off training. You have to accept that no matter what you want to do in BJJ, you're going to spend the first 6 months or so getting tapped out. What you've got to focus on is defense initially: staying out of dangerous positions, escaping attacks, getting in to strong positions. That, IMO, will serve you best long term.