Don't feel bad . In the beginning everyone faced the same problem. I also faced the same problem 6 months ago , but now I am Expert on codeforces (Rating 1647)
If you don't feel comfortable while solving problem A. Then do solve around 30-40 (rated range 800-1000) problems on codeforces. Most of the time we have to think of O(1) (Time complexity ) solution in problem A but sometimes we have to solve in brute force way.
If you feel stuck while solving problems , then read editorials.
For solving Problem A: You have the knowledge of basic Maths and sometime greedy solutions. Always try to think the solution in 4 to 5 minutes
For Problem B : Prefix Sums , Greedy(Multiset lower_bound function is important ). Do solve around 80-90 problems (rated range <=1400). Always try to think the solution in less than 40 minutes
For Problem C : DP, DFS,Graphs, Trees,Strings .
Give virtual contests thrice a week.
If you have a friend who is good in CP , take his/her advice and discuss problems that you are facing during contests with him/her . This will help you a lot and day by day you feel comfortable in solving problems A,B,C.
Solve as much problems as you can , give all the contests on codeforces and codechef.
Give at least 3 hours a day for CP. Solve at least 5 problems a day.
Be consistent for 2 to 3 months and then you will see a huge change in your problem solving skills.
Always stay motivated . I hope this will help you , I am new at hashnode , I joined it today