2002 pontiac Grand Am SE

Im a women so bear with me please 🙂
Okay, so i bought the car 6 months ago. For the first 2 months it drove beautifully. Then the bad upstream o2 sensor came on and I lost about 5 mpg. Replaced it, 2 weeks later a new code popped up, bad catalytic converter, and more mpg loss and power loss. I just now got it replaced (The last guy (illegally) put a pipe through it.) I had not known this until replacement. The lost hp is defiantly back, she sounds great. Although I am literally getting 10 mpg. Its been a slippery winter Michigan but something is wrong. And when I fully stop at a light, foot all the down on brake, the car tries to lurch forward. It is a tiny little jump and could have gone unnoticed, I’m not sure.
The air filter is good, the spark plugs are new, it needs a oil change soon. Everything else is mystery.
Why does it jump forward?
Why did the car run so good if there never was a catalytic converter?
Why is my mileage so terrible?
I have the Haynes manual and no car lifters, I can squeeze under the car, but there’s no room to work.
Please and thank you very much!!! I am poor and have no one to help me with car stuff so I don’t want to go and have a repair guy lie to me.