I mean...you're rated higher in blitz than you are in rapid. Right there, that's a huge problem.
i am not playing blitz much bro that's why its rated higher, cuz i can beat 1500 players too sometimes, i am mostly playing rapid so its less 
and did you see my last rapid game i had a missed win i didn't even noticed it
"i play winning in the beginning and drawing or losing at end"
++ a won game is the hardest game to win - Lasker
"i am playing very carefully" ++ not careful enough apparently
"sometimes i see a great tactic and the i sac my piece and then i mouse slip, or i missed a way that my opponent can defend" ++ stay calm, do not get excited about a great tactic, always look at all ways your opponent can defend
"most of the time "i lose due to tactical blunder"
++ Always check your intended move is no blunder before you play it
"do i lack opening theory" ++ no, certainly not
"or endgame theory" ++ that is useful, but not a priority yet
"do i suck at calculating?" ++ maybe, but start with blunder checking first
"i play winning in the beginning and drawing or losing at end"
++ a won game is the hardest game to win - Lasker
"i am playing very carefully" ++ not careful enough apparently
"sometimes i see a great tactic and the i sac my piece and then i mouse slip, or i missed a way that my opponent can defend" ++ stay calm, do not get excited about a great tactic, always look at all ways your opponent can defend
"most of the time "i lose due to tactical blunder"
++ Always check your intended move is no blunder before you play it
"do i lack opening theory" ++ no, certainly not
"or endgame theory" ++ that is useful, but not a priority yet
"do i suck at calculating?" ++ maybe, but start with blunder checking first
umm thanks bro but i play the way you said, i am always trying to find if opponent can defend it and the only i start attacking but then opponent always surprises me with another move where i feel i just blundered , sometimes when i see a move i see that my piece can move to certain squire , so i check him and then i realize i can't move there cuz of the pin, or the knight can't go that far, but thanks for the help , i will try to do careful calculations, 
sometimes when i see a move i see that my piece can move to certain squire , so i check him and then i realize i can't move there cuz of the pin, or the knight can't go that far, but thanks for the help , i will try to do careful calculations,
You're far higher rated than me, but right there you've kind of identified calculation issues.
But as a kind of left-field suggestion, that helped me a bit... Go on that other chess site as a guest and pick up anonymous games. Play a really dodgy opening and THEN try to win. I found myself becoming more resourceful in attack and defence.
i don't know why i can't hold on a winning position, almost every game i play winning in the beginning and drawing or losing at end, i am playing very carefully but for some reason i think chess is not my thing, sometimes i see a great tactic and the i sac my piece and then i mouse slip, or i missed a way that my opponent can defend, i sometimes wining against strong players but sometimes losing, most of the time i lose due to tactical blunder, if i play passive it always a draw, if i play aggressive sometimes winning or winning and losing or draw at the end, pls can someone go through my games played and tell me how can i improve my gameplay, i really love chess and want to get better, am i missing something? or do i lack opening theory or endgame theory or do i suck at calculating? pls can someone help me