Online casino pattern

I have been trying out a few online casinos in recent months.  I am losing but not a lot because I only bet small amounts.  Recently I have played slots, video poker, and blackjack at Lucky Red, Royal Vegas and Bet365.   The pattern so far is that I win several hundred dollars at the beginning and then hit an unstoppable losing streak until my funds run out.  I usually get a welcome bonus when I deposit the initial funds and therefore cannot withdraw any winnings until I play through 30-40X my initial deposit and bonus.  I wonder if I didn't take the welcome bonus, I would win initially and be able to withdraw winnings.  Or am I allowed to win initially because I am imprisoned by taking the bonus?  Is there a switch that they pull after the initial winning streak to put you into the inevitable final losing streak?  Has anyone had similar experiences or am I just very unlucky?

The key here is to avoid all online casinos. I hit a big jackpot playing 3 card poker about 10 years and got all my money and never played again. The pattern you described is pretty standard. Very difficult to win over a long period. Take your money to a brick and mortar casino of you like playing casino games