Welcome to BadmintonCentral! The general feel I get when watching you play is that your rhythm is 'backwards'. By that, I mean that when your opponent hits a stroke at you (especially to the back), you gently move to retrieve it, then you rush back into the middle, often rushing so much that you still have forward momentum when your opponent hits it to the back and you struggle to move back again. The 'right' way is to recover gently and suddenly accelerate to reach any stroke, even if it's a really high clear - the quicker you get to the shot, the more options you have. You want to gently recover so that your muscles are not tense when you suddenly accelerate, and critically, that you are balanced. It is better to not recover to the centre of the court and be balanced, than to recover to the centre with forward momentum! I highly recommend the Peter Rasmussen drill - a quick YouTube search will find it. Focus on the rhythm, and as he says, don't go all the way into the corners. Good luck!
As you mentioned in your original post your serves good be better. Improving that would help you a lot very quickly as it would give you more time to react to his reply. If you want to play the high defensive service then practice to ensure that when your opponant replies his right foot is on or preferably behind the rear court back line, you seem to be serving around 3/4 court making it easier for your opponant and difficult for yourself. Just take 1 step at a time It's a great journey so enjoy it
Not feeling like giving a lesson when I am myself a beginner but I would also recommend watching videos of different pro players. It really does help. By watching some matches over and over (sometimes the same) you can pick up a few things here and there. I personally progressed a lot by watching some matches racket in hand and trying to copy the body movements/footworks of the players.
