Keep studying, keep playing online, try to play OTB games with strong players. I would suggest playing against a strong computer, and also try to play as many 60+minute games, so you can have time to train your chess muscles. Do chess drills, like concentric square (de la Maze) and tactics puzzles. Read chess books. Keep going! You can become a strong player, you just need to believe you have what it takes to do it. You may not have access to many OTB tournaments but if you desire to become a strong player, you will find them.
Should I stop to play?

Should I stop to play chess? I can't play many OTB tournaments , so I will never become a strong player, plus OTB players say that playing online isn't serious and only OTB is serious chess. What should I do?
Depends on your objectives. There is a strong correlation between the amount of serious OTB chess games you play and your chess strength.

It's the same game, only the interface is different. The OTB players might defend their preference in this way, but don't let it discourage you.
I believe you can become strong by just playing digital chess. Though I prefer OTB (ratings are more acurate, the game is more "personal/real" if that makes sense), mostly I play online because I don't have the luxury of playing OTB a lot. The learning curve will probably be roughly about equal, depending on time controls and frequency of playing (And maybe the "wanting to win" feeling you get from OTB more than with online chess.
Also becoming a strong player isn't all there is to the game. It would be very nice, but an inability to do so isn't any reason to quit playing (perhaps it should be the opposite).
If you want to become strong by just playing online, take any game as serious as if it were OTB. (I wouldn't know though, but it seems like the best I can do for giving advise).
Should I stop to play chess? I can't play many OTB tournaments , so I will never become a strong player, plus OTB players say that playing online isn't serious and only OTB is serious chess. What should I do?