OP - whatever the 'personal' reason is, if it's not that he won't or can't, or that you won't or can't, then you really should just do a quickly registery office job rather than change your name if it really matters to you to have the same name now, not just wait until you are married.
Once you change your name, people will ask straight away "oh, did you and DP get married?" - and you'll have to explain that no, you just changed your name to your DPs. At that point, most people will say "oh right" or possibly ask you why, some might even ask why you didn't get married, but most likely, most will walk away feeling really sorry for you that you clearly want to get married but your DP won't marry you.
Woman who are co-habiting and happy about it don't go round changing their names to fake being a legal unit. Many woman have a different surname to their DP/H - sometimes it's that they are unmarried and happy about it, sometimes it's that they are married and didn't want to change their name, sometimes it's because they are married and at a stage in their career where changing their name would cause problems.
Changing your name obviously means a lot to you, it's clearly very symbolic of being a family, you might feel a little cheated when you do have the big wedding that it'll be Miss Chicken [DP's surname] marrying Mr [DP's surname]. There'll be no lovely moment when your DP or someone else says "hello Mrs [DP surname]!" at your wedding, because it's what you already are. There's no sending off your wedding certificate to change your name, because you'll already have done it.