Can someone help me figure out why intimacy always dies in my relationships?

I've had intamacy issues in all of my long-term relationships, seven to be exact. (There's been one exception to the rule - the most recent relationship. We broke up a month ago. My ex was 100% the reason we were no longer physical. It happened very early, after about 6 months. We should have still been in the "can't keep our hands of each other" phase! But I respected him and didn't push the issue for 2 reasons. #1, I knew there's was no way harping would help. #2, I figured we'd end up that way eventually based on my track record, and part of me was happy it wasn't my fault for once. We got to that point in time a lot sooner, but for once, I couldn't be blamed!)
I'm to blame more than the other six former partners. In each relationship, intimacy stopped, usually about a year to 18 months after we started dating. Nothing specific happened to cause the change. It wasn't lack of interest or attraction. Love remained, but no physical expression of it. It wasn't an abrupt change either. I realize physical intimacy naturally dwindles as time goes on, but it shouldn't stop all together. I don't understand why it keeps happening, and I admit I'm 90% to blame in each case (except the most recent.) I can't even begin to TRY to fix this if I can't figure out the root cause. None of my past partners were all that similar personality-wise and no one relationship was enough like another to recognize an obvious pattern. Over the years, I've learned to choose more compatible partners, but that makes the break-up that much harder. I don't feel the need to get married, I just want a long-term relationship that doesn't end. Commitment and loyalty to each other, for the long haul. I'm officially middle-aged tomorrow. I'm worried I'll never be able to have a successful relationship.
Can someone help me figure out why intimacy always dies in my relationships?
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