Hello Zeffke,
What about your BR. Guess you had to start again from small-stakes? 
Poker is actually a lot of fun and I do like competition as well. :> In poker it's even harder to beat since a lot good players are around on higher limits. Everyday you learning something new.
Probably a lot. One of the biggest leaks: I don't know what to do vs. flopraises. For example if i call a raise, i easily toss it away on the turn because I can't see me win the hand (burning money), etc... --> solution: post hands!
Wouldn't say it's always about 3betting. What did help for me is that you have to figure out the opponents range. If you know the opponents range then you also will know what kind of play you can do against him. What kind of plays would be the most profitable plays. Like either maybe to get it in if we have correct equity. Or is it just safer to fold even sometimes you might fold the best hand. Different boards, different opponents. But of course, the main solution is posting hands and discussing with others. How else you want to figure our if you are doing great with the equity?
Not enough 3-betting might be one as well. So when I 3-bet everyone knows I have a monster.
On the live tables it might not come up so much. I mean of course people still will realize that you are 3betting a lot. But in online it's whole different story. People do have stats and if you don't balance your 3bet on higher stake and do it only for value then it will be the case that you wont get paid when you have a strong hand. Wouldn't say that you have to turn yourself into 3bet maniac style. But still pick the opponents against you can 3bet, like if he is folding a lot to 3bets. Or other case would be that if you have a great hand which has good equity postflop. Or as well even a very easy to play hand like mid-PP, you mostly gonna just hope for a hit and if not then will give up your hand. Sometimes even Check/Folding directly on flop.
Or another one playing hands like KQ AJ KJ... if I call a raise preflop with them I have a hard time postflop. While 3-betting them things might get ugly as well.. Solution --> post more hands & watch videos/coachings
One good solution for your calling with those hands. I am pretty sure that we are talking about calling 3bets here. Try to figure out once again the opponents 3bet range. If we do hardly dominate his range and we are doing perfectly with our equity against his range and may even see worse hands like for example if we Call with AQo we will also see ATo then we can do it profitable. If we rather expect not to have that much information on him then rather just fold the hand and don't try to be fancy. Sometimes it saves some money, not much but at least something. 
Hopefully it helped, I am up for any discussion if you want to.
Welcome to the Course and Best of luck! First points earned.