Assalammualaikum anna. how do you managed to work part time and study and still manage to get good result?

The only way to be good at managing your time is by knowing your priorities. Always know which one comes first, which ones goes second so if one dah you'll have to decide between those two, you'd know which one goes first.
Despite your business & lack of time to do everything perfectly, always do everything perfectly. Be a perfectionist. Do things wholeheartedly, your assignments, your work, complete them like your life depends on them. The blood & tears will be worth it in the end.
Do not give up.