@FinallyOnTrack has some really good advice about repetition to get reaction mechanisms down.
At the end of the day, it's
all about
electrons. In the past, it's helped me to understand the basics first: for example, a carbonyl bond is basically a huge dipole, suctioning electrons towards that electronegative oxygen. Now the carbon is almost completely robbed of electrons (electro
phile), and is prime for attack by an electron-rich nucleophile. You'll see patterns like this over and over again. For example, you can take almost any reaction in that reaction map
@FinallyOnTrack attached and try to predict where the electrons want to be. This helps explains things like the fact that electronegative atoms like Cl make acids stronger by stabilizing the electrons / negative charge when the acid donates an H+. It also explains why some groups are added at the more substituted position, and some at the less substituted position, depending on which is better equipped to handle excess positive charge as an intermediate.
Something else really common you'll encounter is redox. Your weak oxidant is PCC (an alcohol can gain just ONE more oxygen bond), and all those chromium-containing oxidants are strong oxidants, which can oxidize all the way to a carboxylic acid. Your reducing agents have lots of hydrogens (LiAlH4, NaBH4), so when you see them, remember that hydrogens are just protons, so they'll be looking perhaps for some electron-rich double bonds to disrupt where they can add in a few bonds to hydrogen.
I definitely second the idea of creating flashcards. What has helped me is to right out
very general schematics like "ROH + ROH -->" on the front and "ROR + H2O" on the back so that no matter what alcohols or ethers you're given, you'll be able to predict the reaction like a pro.
