I (31F) have been with my boyfriend (39m) for 5 years, bought a ring together 1 1/2 yrs ago and he still has not proposed. Am I being a nagging girlfriend if I put an "end date" on it?

My boyfriend and I have been together 5 years and we have a great relationship. After a lengthy discussion he and I bought a ring together on an exciting day out in Miami a year and 7 months ago. This ring was 100% an engagement ring. After 6 months of having the ring and no engagement, I asked him what was going on. He had some excuse about not being about to figure out a good way to propose.
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Am I "THAT" nagging girlfriend if I bring this up again and ask for a timeline of when he thinks he'll "have the proposal planned"?
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My issue is- on one side, I don't want to be the nagging girlfriend
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On the other side, I cannot continue to stay in a relationship where I am being strung along. I don't think it has anything to do with the planning of the proposal. He SWEARS he definitely wants to get married.
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TL;DR- Boyfriend and I together 5 yrs. Bought a ring together and we've had it for a year and 7 months. Still no proposal.
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EDIT** Copy/pasting an earlier comment of mine-
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He brought it up (Buying the ring). I was hesitant and I told him my reasons behind being hesitant. We continued talking and he booked a hotel and flight to Miami and had me design the ring. I have NEVER wanted to get married, ever. When I realized it was important to him, I thought about it and decided- yes! Lets do this! This is why I am SO confused. We weren't on the same page because of me. Then we were- now we aren't because of him and he isn't giving me a clear reason as to why.


Source.