when i write things , i tend to write riffs that sound the same as ones i previously wrote ... I try to mix things up , and try new techniques ( i write one or two great riffs , but then lose ideas / my songs start sounding alike again ) . Do you guys have the same problem ?

If yes , what do you do to mix it up ?
I've been having that problem too as I've started writing songs. When I come up with something good I end up making everything I do sound like it lol. It seems to help to just stop for a while when I run out of ideas and come back a little later.
You can listen to music to get inspired, understand what bits you like and what they are doing and try them yourself.
Also maybe doing such things as changing your tone on your amp, if you play with distortion most of the time, try the clean channel for a while. It could open your mind to new ideas
Change the key you're in, write in a different mode altogether, change the time sig, change the rhythms in your riff, etc
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