Check your clutch adjustment and you could change the fluid and see if that helps but 96-98 transmissions are notorious for synchros going out. My 98 does the same thing into 2nd and its the synchro. I am replacing it with a 3650 next week. That's what happens when the transmission has paper blocker rings instead of carbon fiber that the 99-up ones got.
2000 Mustang GT Steeda #0048 34k miles. 99 Cobra 4V swapped, FTBR IRS swapped, exhaust, tuned by James Gordon.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.