In addition to undertaking what Brock has already advised you could also remove, clean, lubricate and replace the gear linkage that connects the lever to the gear selector 'drum'(for want of a better word).
This is quite often over looked with regards to routine maintenance as it does collect a lot of moisture and road grime. If that fails to rectify the difficulty in engauging a low gear when stationary, you may well need to replace the selector drum itself. The ST's selector is inherently 'notchy' to begin with and not helped when hot and old oil.
Reason...IF, because of recurrent forced lower gear selection due to un-lubricated and 'sticking' linkage that hasn't been maintained OR if the bike is overloaded (as with a trailer, for which it was never designed), then often the excess weight (exacerbated further if 2-up), on an uneven road surface adds weight to the drive train that requires the rider to actually 'boot' the bike into 1st gear. Rather than easing it into gear with a steady downward pressure. The selector components burr over and wear beyond repair.
The good news is they are not to difficult to replace.
As for your clutch, l have another thread elesewhere on this site with pics when l changed over a clutch basket that had burred badly and l used the same plates and removed the non OEM anti shudder rings.
Cheers the can:-)