I often get into stupid blunders, get myself into pins and threats that I don't see on time etc. How can I avoid that ?
Learn chess geometries and patterns. For example, if two of your pieces stand on squares of opposite colors (or along a diagonal with one square between), then the pieces cannot be forked by a knight; and if the pieces will remain there for several turns, then they will remain mostly immune to that threat.
Learn more chess geometries and patterns. For example, a bishop is often an eventual threat to attack any foe that stands on its diagonal, even if, say, two pawns standing on the diagonal currently block the attack. If the bishop remains on the diagonal for several turns, then a player can see the potential, eventual threat, all along the diagonal.
Learn further chess geometries and patterns. For example, if your opponent has only one bishop, if you have chosen a square of the opposite color on which to place your rook, then the problem that the bishop might eventually capture the rook is less worrisome.
Observe that items 1, 2 and 3 are all positional in nature. An opponent of intermediate ability—say, ELO 1500—may see less of the board than you think; but if he or she has positioned his or her men on squares on which it is hard for you to develop threats against them, it can seem as though the player sees the board in a way you do not.
Know where your hanging (unguarded) pieces are—knights, bishops, rooks and queen. Pay special attention. Whenever you have an unguarded piece, the opponent might develop a tactic against it. That's okay. No chess player can guard all pieces at all times, but if you cannot hold the entire position in mind at once (I cannot, either), then you can at least hold certain features of it in mind. Hanging pieces are often an important feature.
Avoid wasting tempos. When you make a pointless move, this usually affords your opponent valuable time to exploit weaknesses in your position. When all your moves are purposeful, by contrast, your position might still suffer weaknesses—even weaknesses of which you are unaware—but your purposeful moves may deny the opponent the opportunity to exploit the weaknesses insofar as that your moves have forced the opponent to defend rather than attack.
Much of my answer is indirect, but you can see the board better when you have arranged the board such that there are fewer things on the board that are necessary for you to see. This is what positional play is about.