It's actually not the mountain backpack, it's the tactical vest. I had the same issue. Try taking off the vest.

I can't play in first person. I just can't. I've tried. I have so much difficulty navigating through buildings and around obstacles without being able to actually see what my character is doing. Sure it gives an unrealistic advantage to those who use it, but the fact is that EVERYONE has the option of using it, so the advantage is not unfair. What I think they should do is make hardcore servers that force first person like in the mod. That way people who like third person like me can continue using it on veteran servers while people like you can play hardcore servers. Everyone wins! Just keep in mind that this game is still in alpha development, so try not to get too carried away with your demands.