Please help me out, loosing hope already. This problem caused me lots of trouble, especially because I couldn't prove it.

Starting in May 2015 I started to get those health problems.The main symptoms were weakness, exhaustion, dizziness and strong concentration problems. I went to the hospital and the result was 31,000 PVC's per day, so I started taking beta blockers. I recovered physically completely in two months, but was only at about 80% of my concentration ability. I started university (easier degree due to this problem), but my concentration didn't improve.

End of October 2015 I started experiencing those symptoms again. They became stronger and I got two colds within less than two weeks. I did several medical tests, which were mostly fine. The only problem found were excessive PVC’s (but less than before). On November 12th I started to feel a bit better, so this and the following day I did a decent amount of work. After that my symptoms worsened. It took me again almost two month to recover physically, but my concentration ability until now varies between 60-70%. I am currently in a study break due to this problem.
At the worst state I was almost unable to read anything. When physically well, I can do simple office type work and read normally. However, I am not able to comprehend the meaning of difficult texts. If I try to concentrate too much, I get tired easily and sometimes dizzy. This makes it impossible for me to write essays for university, it's difficult to understand the news and generally it's hard to advance at all. (But I don't have much trouble using already acquired or known knowledge, such as writing this text or discussing already studied topics).

Extra details:
- Eat healthy and good appetite (might have slightly bad absorbtion)
- Do exercise, but not in excess
- No coffee, little tea (due to caffeine)
- Suffered for three years frequent chronic gastritis (in this time this didn't occur)
- Occasional light dizziness occurred long before May 2015
- Most doctors clearly stated PVC's wouldn't cause such concentration problems
- Did MRI, all possible blood tests (even HIV and most hormones), but all good
- Since 5 years, drink a lot of water. Most was 2.5 years ago, when I was so thirsty in summer, I often drank up to 10L a day. Currently ~5-6L in summer, less in winter.
- Before May, was top student with a bit above average memory
- Take things close too heart, which makes difficult situations a bit more stressful
- PVC and concentration problem worsen under stress