Don't be ashamed of that. It happens to a lot of fellow CPers, including me. The key is to implement the simplest solution that works to get Accepted. At least in CodeForces, I know that the first problems always is very easy to solve, so my mind is focused on finding an easy trick.

Yes I can definitely help! If you are not happy with your ability to solve AB why don't you practice AB? Solve a B, then analyze where did you overcomplicate it. Repeat.

tl;dr

If you have a problem with trying to overoptimize a solution during a contest when you don't have to, you can look at the time and memory limits in the statement and check if your solution fits within them. Take the worst case inputs (usually the maximums), plug them into your code and approximate how many operations (loop iterations times the operations in them) your solution would take, Codeforces can execute around 10^9 operations per second (https://mirror.codeforces.com/blog/entry/77090). You can also run your solution on a worst case scenario locally and check if the execution time and memory usage fit within the limits in the statement though that's a bit slower to do.

It's important to remember that methods to solve problems result from observations about the problems. So if you find yourself trying to pattern match a solution to a problem, that's not always the correct way to go about it. You should be making observations about the problem itself, which in turn guide you toward a certain type of trick/method to solve it.

Yes, I too have the same problem.(I have 1710 rating, and this is my alt account for giving div3 :P).I mostly solve C and struggling to solve D.Sometimes I solve D,while sometimes I can't.However,I take so much time to solve A,B.Just as you said I overthink A and B.So much that I once solved div2A with segment tree and took 30 minutes,which was just a simple implementation problem. In the same contest it took me 10 minutes to solve C.