Tactics

I suggest not only doing the tactics trainer here and at chesstempo.com, I suggest that you ignore the time pressure on the tactics trainer here and spend at least 5 minutes trying to solve each problem if you don't see the answer sooner.

Then, after you have solved or failed the problem, look at the TAGS describing which Tactical Motifs were used.  Learn the motifs you do not know from the interactive pages listed below - eventually you should be able to name and describe all of them, which will improve your ability to see them:

https://www.chess.com/article/view/chess-tactics--definitions-and-examples

https://chesstempo.com/tactical-motifs.html

Then, go over the problem again and ask yourself why you did not recognize the tactics needed sooner.  Do you not easily recognize the patterns involved?  See the pattern and try to look for it in your future hard-to-find-answer problems along with other patterns you learned.

I returned to chess in 2017 after a long time away.  Below is my 2017 chesstempo rating chart. If this 67 year-old can improve like this doing an avg. of a few problems a day, you should be able to do even better IF you use the above learning plan and goals (yes, I have an Advanced Teaching Certificate in the USA and am an expert in devising methods of achieving learning goals):