Many people have said not to trust dopplers for reassurance once movement can be felt (and rightly so), so I wont echo that too much.
I use one, I have been using it since 8 weeks (to anyone reading this thinking about trying to find the HB at 8 weeks, go to youtube first, there are ladies who show you exactly how to angle it to pick up the sound to early, and you also have to have your doppler at full volume, and once you find the heart beat, don't linger on it for ages, just reassure yourself for a minute or so then turn it off), because I have had a MMC before and found it very reassuring. I am not one to panic quickly if the HB eludes me though- so not finding it quickly didn't cause stress.
At 15 weeks baby is still pretty adept at hiding. I suggest you go to youtube and find some videos of ladies finding HB's with their dopplers at or around 15 weeks, this will show you how to angle it and such to get the best chance of picking up the sound.
My midwife is fine with me using a doppler because she knows I am aware that it is NOT to be used to reassurance once movements can be felt, and she agrees with having some reassurance for people who have had MMC's while in the very scary first trimester.
I'm 31 weeks now and actually still use my doppler for a minute every day, but this is because I'm really enjoying knowing what my baby's "normal" is regarding their heart beat and it's like a little bonding thing. One time I even picked up their hiccups on it! Gosh that was so cute. Often times baby is awake while I'm listening and I'll also hear the whooshes of movement, and sometimes the doppler probe gets kicked!