Tip: Pick up the pieces
The biggest benefit of training tactics is developing your ability to solve problems (to find solutions) and to learn and reinforce your memory of tactical patterns so that you will recognise them more easily in your games.
So if you get stuck and find yourself staring at the computer monitor or the page in the book, set up the position on an analysis board and move the pieces around, instead of trying to see everything in your mind.
It is better to find the solution by moving pieces around and trying different ideas than to give up because you cannot see the variations clearly enough. It is true that you cannot do that in a game but that is why we train, to improve our abilities.