Hi Tapani,
I'm only a very low-ranked 4k (who lost like 18 out of her last 20 games :'D), but I'll give it a try since I'm trying to fight the same problem.

I'll mostly comment about the opening because that's where you made the crucial mistakes that cost you the game.
6 - this is usually a move to reduce a moyo / double-wing formation. I haven't seen it played this early in the game, either. I have a strong urge to tenuki here since your corner is strong and he can't do anything horribly to your stones there with just one move. I'd approach one of white's corners now.
15 - Yes, O7 would definitely have been better.
16 - Game estimator is complete rubbish 90% of the time and this early in the game even more. I strongly recommend not to use it. It's indefinitely better to learn to count yourself, thatt'll make you much stronger! You don't have to be absolutely exact at your level, just comparing the size of territories your opponent has staked out with the size of your territories (and counting in komi) is sufficient to give you a good idea of how the game is at that point.
17 - This is a good move but it's also very passive and slow. Approaching a corner of his is much more urgent - how about upper left corner, around F17? Splitting the left side is also a nice idea. If he invades your right side you shouldn't be too sad since you'll give him hell thanks to your strong stones in the lower right corner and will be able to build new strength and/or territory while doing so. >:-) Always remember that strength is not there to build territory but to fight.
19 - Definitely too passive. his framework is now bigger than yours so you have to expand yours and/or hinder his development. Approaching upper left corner ist very big and urgent!
20 - Greedy overplay. Rejoice and be happy as he willingly asks you to make his life hell from now on. I like your answer here too. R14 is very close to block so he won't be able to make a base easily since you'll block the bottom and chase him out. I love this one sentence In-Seong Hwan once said in Yunguseng: "If you shake your opponent, points will start falling out for you to grab."
23 - I don't like the hane/atari since it makes white strong. I'd just nobi at R9 to help the weaker stone (it's usually the one that'a closer to the botom/corner) and then he'll have a hard time settling. He'll either give you much territory if he hanes at P11 (which he wouldn't do) or give you a nice shape if he plays the other hane to escape with his invasion stone. Either way I cannot see a bad result here for black. The white group'll be suffering from bad shape and lack of freedom to develop all the way to safety, only running but not making any points.
28 - In contrast, white now got a superbly strong shape. K16 wasn't bad, but white now has strength radiating throughout the board and it becomes urgent that black invades somewhere asap. At this point I prefer white already.
29 - Way too passive and too small again. You have to do something about that moyo white built and invade now.
31 - Good idea to ignore his move (don't really get it either, maybe he still thinks his group weak). I'm not 100% sure myself where to invade, there are many, many options. Maybe a stronger player can give a few ideas here. I'd have played a bit more from the side to destroy more potential territory, though, like F3 for example. But since white has a good position on the lower side AND on the left, it's really not easy for black.. Invading the corner gives white superb strength to math for his wall on the right side, too, so the bottom area is almost territory now.
43 - Wrong direction. I'm sorry I cannot really give you another option but here you give white yet MORE strength, this time towards the left side. The result you came to is actually a joseki, but it's a bad result for black. All in all black has now lmost no strength towards the center and "only" more or less secure points. White has a strength that radiates throughout the board and will assist in any fight that will happen from now on. This is, in my opinion, black's biggest weakness. Go is a game of balance and you should strive for territory as well as influence/strength. Of you only play on the 3rd line, your opponent will be in a much better position to fight if you ever have to come into his area of influence.
70 - This result is - as you know yourself - horrible for black. You must play O17 and not R15 to prevent your top area from crumbling like in the game. You played N17 to strengthen your upper side but it wasn't able to help you a bit in this fight. Actually, it's like you passed when you played N17. Although I must say that white took pretty good advantage of this area.
73 - Yes, your invasions should be living, too. However, it's almost too late now. White is strong everywhere and you'll have hell making live in this iceland. When you invade you should always
a) have the possibility to make 2 eyes, for example by poking at some of your opponents weak spots
b) have the chance to link up with your friendly stones (strength helps a LOT here - imagine e.g. tengen would be a black stone and how this would change the game)
However...
75 - is in my opinion a great move! If you had started with it and THEN played 73, I see some small chances that something *MIGHT* have worked. Sometimes the order of the moves is very important.
Your following reductions are all quite nice, but unfortunately it's a bit late already..
119 - That's too much. I can see no possible way for it to live.
Like I said it was your opening that cost you the game here. You played almost axclusively on one side of the board during the first almost 30 moves - that's just WAY too much. White on the other hand was able to stake out much larger areas of influency on the rest of the board which helped him greatly in the fighting later.
I think it's most important for you to remember that you just cannot keep everthing you stake out in the beginning. But, your opponent won't be able to do that, either - no matter how bad you think yourself at fighting. I don't think that you are worse at that than others at your level, but at least in this game you lack the support of your stones.
I'd like to offer you this advice:
1. I think it would be very good for you to read a chapter or two in Opening theory made easy by Otake Hideo. It's a really good book to read about the opening as it explains the different levels of urgent moves in the opening.
2. I know myself how hard and unhelpful this is, but: BE. MORE. CONFIDENT. There's no way your opponent will always get the better of you! You have a good sense of direction, this will help you. And also: Don't focus too much on your own weaknesses and (potential) mistakes. Your opponent also makes plenty of mistakes! If he would make less or less crucial mistakes, he'd be a lot stronger than 8k! You should watch out more for your opponents mistakes & weaknesses, gather all your courage and "kindly" point your finger there. I'm absolutely sure you'll have some good results then. For some practical tipps I'd recommend Attack & Defense by James Davies. It's one of the best books there is out there and you'll learn tons from it. You won't be abble to get everything by reading it once, but this is a book that'll walk the way to 1d with you.
I hope that my comments were a bit useful... Keep at it and good luck!
