i dont know if im qualified to give advice but you should probably start out with bfs, dfs and dp(because these are used very often i think, like the use of dp in todays D problem)

You could've asked this from your main account. There is no need to feel low or ashamed if you don't know enough.

It's very normal. This was just me a year back. I often would get really furious for being stuck for 3-4 months and then after 1.5 months I was blue and even I was suprised that I could get so well in so short time. No I didn't do anything special or learn any advanced thing. Keep patience, keep practicing and it is an automatic that you will experience improvement in near future.

From my experience, I believe 1600 rating is consistently solving Div2A-C.

Div2A and Div2B are usually ad-hoc or greedy and have small bounds, so using any advanced data structure is likely overkill. I think there are no tricks to solving Div2AB except practice and familiarity with the types of problems and the language you use.

For Div2C, more complicated concepts such as graphs start to appear and I think it's good to know how to traverse graphs (dfs and bfs). dsu is also very useful yet simple but I don't know why it's not as popular. Other than being familiar with dealing with graphs, I think binary search, prefix sum array, classic algorithms such as kadane's, classic dp problems, and maybe ternary search and fenwick tree will be enough.