Although it usually doesn't take long to begin to suspect the awful truth when you're in the wrong relationship, it can be difficult to admit to it and find the strength to make the tough decisions. There's always that little voice of hope whispering that maybe you could make it OK, not great but OK, if you changed something like... moved to Mars, became someone else... Yep, usually you just know. However, if you need some help to reflect, here's my thoughts from clinical experience and research to help you recognise if your relationship is likely heading to Misery Town.
When it's not right there's a flatness to your life and perspective that you didn't have before. If being around your partner regularly gets you down because they're habitually critical, unsupportive or you just don't share the same basic values and perspectives about life and love, you may be in a dysfunctional situation that will be difficult to improve.
You rarely want to have sex although you can see your partner is an attractive person -- they're just not that attractive to you. You might be barking up the wrong long-term tree if you don't feel as passionate as you know you have in other relationships. If it isn't hot at the start it's unlikely to get much hotter later.
You feel ambivalent about a future together. Life is for living passionately, not for just dragging yourself through the days. It is possible to find both gentle pleasures, friendship and passion in the one relationship. Settling for what you consider a mediocre relationship is rarely a lasting formula for happiness.
When you disagree there is little respect for one another. This is a big one. Research tells us that it's how couples respond to one another under pressure that either bonds them as a team or drives a wedge between them. Anyone can be easy and happy when there's nothing wrong, it's when the pressure comes on that you show what you're really made of. If you can't be compassionate and loving under pressure it doesn't bode well for a long future together because life isn't always simple and happy and you're going to face challenges at some point.
You fantasise about leaving. The writing is probably on the wall my friend. You're a fair way down the exit ramp when you're thinking like this.
Your life doesn't feel better because they're in it. Long-term couples who are still in love after decades of facing joys and challenges side-by-side often say that their lives feel immeasurably enhanced by each other, and that they've helped each other to live their best lives.
Ask yourself: Is my life better because this person is in it -- not just because I'm with someone, but because of this particular someone? Your answer will help steer you on your authentic relationship path.