Serious doubts should be taken very seriously.
Reasons can range from incompatibility of hashkafah, insensitivity, difficulty communicating (“not on the same wavelength”), to indications of serious emotional instability. In one case I know a girl sensed something was off but could not find a tangible reason, got married and divorced – the husband dropped his mask once they were married.
On ending a shidduch the other party might be very distraught, or perhaps also feel relieved – depends if they also sensed there was something amiss.
If in doubt, talk to a trusted Rav or Rebbetzen or try something like a Shalom Workshop. If the other party is not fully willing to participate, enough said …
Do not throw away your life.