I have been struggling with the way I hold my racket. The reason - a problem finding a grip where I can get a ball feeling. And when I find it, I can easily lose it if I put the racket somewhere and pick it up again later. Moreover, there are times when I cannot find a good grip whatsoever. At such times, I simply do not know where the ball goes after my stroke.

I tried many different things, holding the racket more up, or down, have a tighter, or more loose grip, etc.

I hope someone can help.

I think as long as you're using a standard grip, you should be fine. Are you able to take a photo of your grip from the front and back?

It is a rather common shakehand grip that favors backhand a lot. I begin playing by holding it loosely, and in this way I usually find some good feeling, but as I play, somehow the feeling goes away. Then, in frustration, I try to hold the racket differently, as my head keeps thinking about how I hold it.

I have resumed playing a few years ago, after about 15 years of staying away from it. However, I had several month, 2-4 month breaks, during that period. Currently, I play at least 3-times a week for 3 hours/day

As a kid, I remember changing the grip a lot depending on type of stroke, and I hand a great ball feeling. I was a classic defender/hitter, mostly finishing the rallies by a quick backhand flick after my opponent gave out an opportunity. 

Nowadays, I am playing close to table, using fast and placed backhand strokes, and spinny forehand loops. 

Usually I lose my forehand feeling first, then, any stroke in the short game, service return, becomes unpredictable. 

Sorry I can't be of more help. I still don't think the grip is likely the issue. If at first you're feeling fine, could it be because while you're warming up, the balls are generally more predictable. And if later on you're doing more match play or match drills, the unpredictability of the ball is causing you to tense up and miss.