You are clearly taking the GMAT seriously, which is great. It sounds like you need to boost your knowledge of the strategies, practice a bit more effectively and tie everything together with more Sections and CATs. Here are a few of my thoughts on your past prep:
"l got very frustrated because I kept getting lots of questions incorrect."
- The GMAT is a tough test and your attitude during prep is really important. The key when you miss a lot of problems is to really learn from them during review and to stay positive. To some degree, missing a lot of problems may indicate that you have not fully learned/integrated the strategies. If that is the case, then you need further review there and it is important to embrace that. Sounds like this is something that you rectified fairly well later in your prep.
The
MGMAT curriculum has no gaps, so again, if you felt there were gaps in your knowledge, that was likely due to non-ideal learning of the strategies and/or not enough practice to fully integrate those strategies. More on this in a moment.
Taking breaks in your prep, which you did, really hurts progress. This next time around, plan on taking no more than a day off in a row. You need to stay in the GMAT mind state to really make strides.
I think your target score of the high 600s is reasonable. As far as materials to use, a plan to follow, and guidance on getting the most out of each prep hour, I recommend
GMAT Clarity. My guide should really help you integrate the
MGMAT strategies so that you see major improvements on both sections.