At a deep, internal level, you're afraid of progress and change, and the result of this feeling manifests itself in an interesting, seemingly contradictory way. Deep down in your soul, you feel so distrustful of change and personal growth that you wind up breaking off relationships prematurely or otherwise bringing about big, traumatic changes, almost as a preemptive strike. You feel more comfortable if you're the one causing the changes; at least then you know what to expect.
After all, if you stayed in it for the long haul and allowed things to deepen between you and your lover, who knows what might happen? They could end up leaving you and breaking your heart, or worse. But that's the risk that everyone takes when they fall in love; if you don't risk that, you're not truly getting intimate or experiencing the power of love. And whether or not you realize it, you want that intimacy; you want to feel that power. If you didn't, you wouldn't try so hard to guard yourself against possible upsets by causing them on your own! Examining your fears would be a much more productive pursuit than continuing to wreck perfectly good relationships.