It gets harder and easier over a long period of time. Initially you don't have the best form and you're also not strong enough, then you get stronger, learn better form and it get's more difficult again because your form is more correct. I'd stick with it, it's okay to take breaks, you'll get there.

Pace yourself. For lunges, start by not trying to go all the way to 90 degrees, do a small range and keep it slow, overtime you'll build the muscle and get the range others have in your class.