i've been playing for around 2 years and am starting to get frustrated because when i try and use a pick, i hit the string and the pick slides out from inbetween my fingers and goes on the floor. because of this, ive played for 2 years without picks. can anyone help?
(BTW, I've tried the 'easy grip' picks and the same thing happens.
put some double sided tape at the top where you hold it, or try hold it with a tighter grip, it will make alot of tension in your arm but youll get used to it
My Rig
Fender Blacktop Strat HH MN > Joyo JF-01 Vintage Overdrive > Boss ME50 > Line 6 Spider IV 30

Other Shit
Jim Deacon Acoustic
Yamaha 12 string acoustic

New Amp Fund
In what way do you hold the pick? Could you perhaps post a picture of you holding a pick so we can see if you're holding the pick in the wrong way?
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It sounds to me like you're not gripping the pick hard enough. On rare occasions, I lose my grip and this happens when I'm strumming on my acoustic.

Have you tried adjusting your strength, position and even angle of your pick?

Edit: Not going to change a single thing, but I misread that you've been playing without picks for 2 years, and are switching towards using one and having problems.
I'll try the double sided tape...

Basically i've been trying to use the pick for the whole 2 years but it seems more comfortable to me to hit the string with the side of my thumb.
I used to have the same problem. Since I've switched to holding the pick between my thumb and the side of my index finger, with the other fingers backing up the index finger (side by side, curled up just like the index finger--sort of like a loose 'kung fu grip'), the pick has not slipped even once. I shit you not
try using a v-pick made by a company called v-pick the material they use seems to allow you to hold the pick lightly and still not slip around. holding the pick tightly will add to much tension in your hand and slow you down.