[Pantera]

First thing I would do is to try double clutching it first. Pull it back into second and then try to put it in first.
If this works better then it just may be the clutch is sticky with so little use. It may be that it is not releasing completely. If it is hydralic you may need to adjust the rod out longer or pump it a few times first and then try it.

If that is not the answer your only other choice is to change the trany grease while hot. It may be bad. If it is low it will shift hard when it getts up to temp.

Last of all you may have to replace the brass on the First gear syncro drum.

Good luck
Pantera