I struggle with this immensely darkling. You are not alone.
I also find the idea that all relationships end sooner or later, as IP noted, to be not helpful at all in resolving this issue for me. First, it's depressing. Realistic and accurate but just not how I want to think about my relationships. Second, spiritually, it's not true for me. I'm pagan and believe in reincarnation. I also believe that beyond death, the entities that were my friends, lovers, parents, family still exist in some form and will interact with the entity I become after death in some way. Everyone dies. However, I believe death is not the end. But I digress. You may find those words more useful than I do.
I have struggled to let go of the idea that relationships must be committed and must be lifelong in intent, if not in actuality. I have struggled to cope with the inherent expectations of that worldview. For example, that relationships have to 'go somewhere' in certain ways. One idea that has helped me think about those expectations is that of the 'relationship escalator'. Basically, the idea is that once in a relationship, it moves in a strict direction ('up' to marriage and babies traditionally). The blog, Solo Poly, has some great posts about implications of the relationship escalator, and what it can look like to get off the escalator. http://solopoly.net/
. It's also been discussed here. I've found the idea helpful in framing my struggles but not in resolving them just yet.
(Solo Poly has some great posts on couple privilege which everyone who is in a couple, or wants to be in a couple should read IMHO.)
My relationship with my boyfriend started as FWB. It's gotten more serious over time. I've found that as it's gotten more involved and emotionally important, then my expectations went up. This has caused problems. I'm trying to examine the feelings underneath the expectations, examine the resulting envy and jealousy and get at what is really going on emotionally for me. Unfortunately, I don't have any other useful advice, beyond seeking the deep personal truth behind one's feelings, reactions and expectations.
It's hard. I'm not enjoying it. So far it's been worth it.