Planning on breaking up with my girlfriend this evening. Any advice would be appreciated.

the fact is that i'm asexual and she is not. we're simply not compatible long-term. i've decided to call things off as a result, sooner rather than later. she's coming over this evening after work and i'm going to tell her then, but i'm dreading it. i have no idea what i'm going to say. i'm heartbroken because i truly do love her but i just can't imagine a fulfilling future together. has anyone here been in a similar situation? what did you say? how did it go? i'm an anxious mess right now...