Thanks to this win I'm now a 10k, I wonder how long it lasts until I fall back down

Well here is the game I just played: I was black and had 3 stones handicap against an 8k, but it was hard to keep up with white.
I don't really have experience with any kind of handicap.
I used my 25 minutes as good as I could and won by 5.5 points, but I made some mistakes and I think here and there are still some useless moves or at least slow moves.
At some points I was not to sure what move is better. I also first thought my group on the left side is dead but later in the game I managed to save it.

Anyways as always I would like to hear some comments so that I can improve my game a bit.

I'm the same rank as you so take this with a pinch of salt, but I think Black 8 is a poor response to the white approach.

It strengthens W and allows her to make a big extension up the side. Better to withdraw or pincer. I would only play that move if I already had the corner enclosed.

Thanks Koroviev and EdLee for the quick responses!
I made some serious mistakes, thanks EdLee for the vars.
I don't really know what I thought at move 30 I see that this is stupid.

After move 11: is there anything I could do to make the white area smaller? Invading was a bad idea but how can I reduce it?
Of course I know at this point of the game there a bigger moves, but later in the game I would need to do something about it.

That's jealous/greedy/unreasonable. Even if W turns it all into territory, it's only about 30 points.
Why do you need to reduce it? Instead of reducing W, how about expanding yours? Look at the whole board.
(Also, remember your C5-D6 exchange helped W get stronger; this is why C5 was not in this situation.)

Like EdLee said, this is too small at this point. The whole board is wide open, so invading or reducing at this point is too small. Later on though, B5, F6 or E10 might be good places to try.

Following the actual game, on if you had played EdLee's suggested hane on move 44, you would have more center influence which makes it easier to reduce. Still, perhaps the best move is just playing B5. This takes away an invasion point in your corner, and if he doesn't respond you have moves like B7.

_________________
We don't know who we are; we don't know where we are.
Each of us woke up one moment and here we were in the darkness.
We're nameless things with no memory; no knowledge of what went before,
No understanding of what is now, no knowledge of what will be.

Just don't avoid playing because you're afraid you'll lose your rank--the longer you wait, the worse you'll feel if it happens.