How to avoid losing after gaining significant advantage??

I play well in the opening and middlegame.. get the advantage and then lose it during the endgame or pre-endgame. How to resolve this issue?

Please help.

If you are looking to improve, you need to answer a number of questions:

1) Is this Blitz (or bullet)?  If so, these games mean total zilch!

2) Do you play over the board tournaments?  If not, and you really want to improve, you must make this adjustment!

3) Do you study any books?  Especially those on Tactics, Strategy, and Endgames?  If not, this is a major adjustment that must be made.  1 Hour of studying a book at a 3D Chess Board (forget e-books, or anything that allows you to click quickly - known as passive studying - you need to be serious about what you are doing) is more effective than 12 hours of internet blitz.

4) While I have not specifically analyzed either position in depth, were you ever better except maybe near the very very end in the second game.  I almost think Black was better for a large portion of it.

5) Do you understand defense, or do you just look at what you have, and not take into account threats.  You seemed to fail miserably in the first one.  Allowing f6 by White was a major problem, and then you allowed it to escalate from there.  Qg5 is complete death!

if you see a good move look again ,you might find a great one.

wow do you realise who you played op , GRIMZOVICH is well respected as a very good chess club player , i envy you ive only faced him once online on another site , but have always admired his chess

you lucky person i hope you enjoyed the game

im impressed you got at his king when it reached h2 i didnt manage that in my game , but he is a really good player as im sure you would agree

answer to initial question is both simple and difficult : play good moves