Fistail,
welcome to GMAT. Part of GMAT is about losing spirit, memmory and plenty of things in your way. There were times when I thought I'd never do it, but in the end, I got a 760 in my second attempt, so don't worry.
A 620 is not a bad score for first try (I got 660 on mine) and you can improve around 100 points in one/two months (depending on your amount of study). Right now, you have the basics, so you have to improve them and control the details that will send you to break the 700.
There are plenty of strategies in this forum. Search a bit and I'm sure you'll find the right one. However, in the end is about practice, practice and practice (and finding the right material).
Good luck!