disadvantage of playing too much online chess!!
to me it seems the main disadvantage is that it actually makes people care about their online rating😕...rallrating is just a number...you might have an 1800 rating by beating lots of 1200s but that doesnt mean you are at 1300 strength even...play to learn...thats what this site is about☺ imo that is
Opening theory is theory, there is one time to learn it, and one time to apply it. It's completely normal that you struggle at first, you just need more practice.
As for tactic, if I present you a position and say "solve this tactic", no matter how hard it is, with brute force, you will eventually get it. Now, in a real game, no one is here to shout you there's a tactic. Being able to find tactics and to calculate lines is one skill, knowing when to pause and calculate is another (and you need to developp both).
Also, while training tactic, the emphasis is often put on the attack, punishing your opponent's mistakes, but in actual play, good defense and avoiding blunders weights much more toward winning, so ideally, before each move you make, you should check that it does not enable a tactic for your opponent, that's the efficient way to use tactic.
You can play daily chess online and set up a real board according to that game - try to find every move on the real board first! It takes a while but eventually you´ll improve: it´s only a question of habit, your 2dimensional computer-screen vision will adjust over time!
Also, real-board playing includes a lot of other stuff like direct response of your opponent (mimic, gestual, even verbal). Psychologically it´s something else. Keep playing and you´ll improve!
Good luck!