How bad is it for me?

I bluff alot especially when sense weakness. I dont know but everytime I sense weak from my opponent. Like their bet sizing. I tend to go on top of them.. Which is sometimes not always successful. Is this a bad trait? How much should I be doing this?

I sometimes win small amount of money or Lose big amount. Is this bad?

If your losing more them you are winning you should stop. Also some will make you think they are weak, kill you on a monster hand.
In 2NL, 10NL you should not be doing to much.

Yea I think I need to stop on doing this..

1. If you lose more than you win doing it.
2. If you are playing at 10NL or lower, because people rarely fold there.
I usually study the table to see who is bluffable, and who isn't bluffable.. THEN i look for weakness.

Once a person has a read on u,u in trouble.they will trap u.

Unless it is a week table with uexperianced players.

Lazmansa

This is absolutely true. In higher stakes, correctly sensing weakness, then moving on it, is a phenomenal trait. However, in lower stakes, even if your opponent is weak, he may decide to call anyway just because he doesn't want to fold.

If you don't make money with it it's bad.
You say you smell weakness by bet sizing.
You have to watch out for this. Small bets aren't always weakness...

As others have said, sometimes a check is weakness, sometimes they are just hoping you will bet at it. (I often check my monster flops if I am in early position... and, boy, do I love the guy who makes a move on the pot in late position. )

It's not necessarily a bad move, that bluff when you sense weakness. But don't do it much, and be very careful putting too many chips into your bluff. Some tables, a mere min-raise is enough to take down a pot. On other tables, a pot-sized raise is needed to show you are serious. Obviously, if you are walking into a trap, you could lose a chunk at the second sort of table because you have to fold to that check-raise.

Oh, yeah-- another point. Don't forget to fold when your bluff is raised. Do not forget it was a bluff. (Some of you will snort at this -- others will know exactly what I am talking about. I know I've done this. I have gone to some lengths to stop that expensive leak. When I bluff, I say to myself: Hmmm, I wonder if anyone has anything? And the bet is that question in chip form. A reraise becomes the answer to that question, and my fold is: Oh, so you do!)

How good are you at sensing weakness? When you do so, is it based on actual evidence from hands they showed down before, or are you just guessing? There can be a huge difference. For example, if someone has bet half-pot before then shown down second or third pair, plus has bet 2/3 pot other times and shown top pair, you can be more confident that his latest half-pot bet is a modest hand than if you have only some feeling to go on.

Also the bet size of the bluff is very important. For example on a NL10 table with short stackers it can sometimes be very profitable to fire a min-bet or min-bet x2 as a (semi-)bluff after the flop misses most of the short stackers' range.