The truth is romantic love — soul mate or otherwise — is nice, but couples have to acknowledge the inevitability that their feelings for each other will move to lukewarm and their love will evolve from less passionate to more companionate. That’s normal, and it’s not an indication of a love gone bad. The fact that you are no longer head over heels is not a reason to be disappointed, nor does it necessarily mean you are not right for each other.
When you feel a passionate connection with someone, enjoy it while it lasts, but keep in mind that it’s more important that you like that person’s company, and that you have things you can share in the future.