Hi! I have the same problem sometimes. So keep this in mind: Never rage/get sad when you loose, just say oh well and onto the next game. Now to give you some PvP tips and tricks I use:
Strafing: When you target an opponent swing your sword fast and move from side to side, if you manage to get a good strafe on your opponent it can be deathly and you will not loose many hearts.
Block-Hitting: Click both left and right and the player will swing the sword in a weird way, they means you are block-hitting. I will tell you the pros and cons.
Cons: You will be very slow when performing this tactic. It can be easily defeated with fishing rod technique.
Pros: You have the chance to take half-damage while delivering the same damage your sword does. With the sprint button implemented in 1.7 it won't slow you down as much.
Fishing Rod: The fishing rod is one of the most OP items in PvP, it has awesome knock back when close so when your opponent is charging for you, use it and he will miss hits at you but you won't. Also If he sets you on fire you can keep him away with the rod and then charge. Never underestimate the rod users!
Flint and Steel: This is the best item against any player, and teams. You just need to learn to place it properly and you might just as well save yourself against stronger PvP'ers. I personally use it on defensive and offensive, most players charge directly so just place fire in front of them before engaging battle that will prevent them from running away and you not taking much damage.
Bow and Arrow: The more you use it the better you will be at it, always try to pull it two spaces back or more because one is just not gonna work. Always have many arrows because they will pull you out of almost anything. When someone is chasing you and is so far that flint and steel won't work, turn around and shoot the guy, that will delay him and he takes more damage.
Saturation Food: Always try to keep the food with the most saturation on your hot bar. Like the golden carrots, those will keep you sprinting forever and you can escape people faster or reach them without having to eat much.
Armor: Always be aware of your armor and the opponent's weapon. If your armor is crappy and your opponent has a good weapon it's a better idea to far range battle him. Also if he is stacked maybe evade him or just attack him in the back

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Dude, I had PvP troubles too! Don't worry the majority do too! So always be positive and aim for the win! I can help you out in more detail if you Skype me! Ilhui.Bravo maybe we can play and I can help you and give you my personal advice!