Ask yourself...
Do you feel drained and unhappy whenever you're in a relationship, no matter how good it is? Do you have any idea why that is?
When you're in a relationship, and you envision the future, does it include the person you're with (whether it includes marriage or not)? Or do they involve you, but not him/her? Do you frequently fantasize about being single or dating around?
Are you in this relationship because you want to be, and you love the person you're with, or because you're afraid of being alone/you feel pressured to be in a relationship/etc.?
If you want to get married, do you think about what married life might be like, or just the wedding day?
Does the thought of any kind of physical intimacy repulse you?
Do you end up (subconsciously) sabotaging the relationship, because you don't really want to be in one?
Have you noticed patterns in your previous relationships?
Are you looking to fill a (real or imagined) void in your life?
Are you avoiding relationships because you're afraid of being hurt?
Do you find yourself suppressing your true self and neglecting your needs in a relationship?
Who are you outside of a relationship?