First things first: more donks do not make you lose
. It might increase your variance, but playing against poor player is not going to turn you from a winning player to a losing player.
Let's look at some reasons why you might win live, but not online:
1. You are not really a winning player live. Yes, it's very possible that you misremember the amount of your wins live, unless you are disciplined enough to keep a detailed log of your live play. Pretty basic human psychology to tend to forget your loses and remember your wins. You "forget" your trips to the ATM and all those tournament sessions where you didn't cash live. Online, you can't lie to yourself: your account balance is right there in black and white, and making another deposit often involves a bit of effort that isn't easily forgotten.
Also, your live game sample may not be big enough to determine if you are a winning player yet. Statements like "I usually cash in 2 out of 3 tournaments I play live" are clearly ridiculous. This is a short term run of good luck, as not even the best players in the world can make a claim like this. But if you have only played about ten tournaments it's not out of the question that a positive variance like this could occur.
2. Live players are actually worse than online players. This has been my experience at least: $1/$2 NL live plays like one cent/two cent online. As for tournaments, if you are not doing research and seeking out a good structure, most live tournaments are much more luck contingent due to small starting stacks and fast blind levels. Without a doubt there is a higher percentage
of good players online that you are likely to encounter as opposed to playing live. Your local cardroom might have a few expert players, but it's likely that you don't cross paths that often due to shear numbers, or simply them playing at different stakes than you. Not so online: there are a lot more players who know what they are doing, and that you are likely to encounter, even at the lowest stakes.
3. Less "distraction" live. You are pretty much constantly reminded that you are playing poker, where as on the computer you have different distractions like e-mail, cardschat (!), etc. Live has it's own distractions, sure (Venetian ****tail waitresses anyone ?) but the point is that you are not going to lose your main focus of actually playing poker. There is a very clear line about "what you are doing" when you are at a cardroom: playing poker. Online, this isn't the case, as you can be multitasking to your detriment. While doing several things at once can help some people and prevent them from playing too loosely due to boredom, it can have a negative effect on others.
4. Lower stakes online. You might not take the game as seriously, or play to your "A" level if you are only playing a $5 tournament as opposed to buying in live for $120.
5. Extra removal from the actual money. It's been said that the guy who invented gambling was smart, but the guy who invented chips was a genius (or something similar to that effect). Betting is a lot easier for most people if you are putting in little pieces of round plastic as opposed to hundred dollar bills. It's similar to it being easier to overspend with a credit card as opposed to forking over cash. Online has an even further removal for the reality of it being money. This can be a positive aspect with regard to poker and making +EV decisions of course, but it can go the other way as well for some people.
skills are needed playing online. It's beyond the scope of this topic to innumerate them all here, but clearly ou have to give up things like physical "tells" skills (overrated anyway, imo), and have better statistical analysis ability when playing online. I think that most people will make a lot bigger deal of this than they should. Pattern recognition (in opponents betting), both live and online, is going to give you more information more often about the strength of someones hand than seeing them twist their oreo a certain way.
7. Failure to make the strategic adjustments necessary. Your opponents
play differently online than they do live. Again, it's probably beyond the scope of this topic to get into all the specific ways this occurs, suffice to say that you need adjust your game to compensate for this.
8. You're not a winning player period. Play is much faster online, and you are able to see so many more hands or play so many more tournaments in a short period of time, that it becomes clear a lot more quickly if you are a winning player of not. Your mistakes are magnified since you will be making them far more often. In other words, playing online gets you into the long run faster, where a more accurate assessment of your skill can be seen. It takes a lot of different skill sets to win at poker, including patter recognition, logical/analytic mind, willingness to continue to study/learn/improve, discipline, etc., etc., etc., and a lot of people just are not going to have the ability or time to be winning players.