Hi! I never used this happy place technique, but if imagining a place you can't reach makes you sad, you probably shouldn't use this for feeling better. Maybe you could imagine a place where you can go later, if not immediately, like your room or another place that is quite easy to reach? So, you'll know that as soon as you can you'll go there.
Also, now that I'm writing I'm thinking that one of the places I could use as my happy place could be PC Maybe it could be also yours?