gearshift extremely hard to move into 1st and reverse

[clark.70]

i have a '70 c10, it's 3 on the tree. lately it has been getting close to impossible to pull the shifter towards me to put it into 1st gear or reverse. shifting between neutral, 2nd, and 3rd is as easy as ever, and there is no grinding or anything strange even if i do get it into 1st or reverse. but most of the time it takes all the strength in my body to even make it budge. it's always had this problem a little bit when it is really cold outside, but it was never even close to this bad. any ideas or suggestions?

i'm just in the beginning stages of learning how to work on my own vehicles so you may have to dumb down your responses for me somewhat. thanks. -clark