Some people just aren't good at relationships. Clara is probably one of those people (men get all the flak here but women can be just as bad sometimes). I believe that she liked Danny. If she wasn't TARDISing around, she probably would've done the whole bit with him, as shown in "Last Christmas" -- they would've gotten a house, owned a cat, gone to Paris, the whole nine yards.^I also never really bought that Clara cared for him, which made me like Clara less. He clearly cared for her but I don't think the way it was portrayed on the show did a convincing job. It made it feel very flat and only existing because the script said it should. It seemed that Clara declared her feeling for him but we never really SAW it in her actions.
*Shrug* Dunno. Maybe it was just me.
But Clara loved being in the TARDIS. She tried the double life...it didn't work. I believe that Danny was her "anchor" to Earth. Not in the sense that he held her back, but that he still gave her purpose to be the "old" Clara, the pre-TARDIS Clara. She cared for him, and she appreciated him, but she never gave him more than probably 60%...because she loved exploring too much. The "Forest" episode showed their differing perspectives, and did it for a reason.
Then when he died, suddenly she lost her anchor. She probably felt guilty as well as realization of how much he probably meant to her, but she never properly told him or showed him...because as she said to him herself in Death in Heaven, she "wasn't very good at it." She lied to him and realized too late that she took him for granted.
That's not to say that Danny didn't deserve some blame too. He was jealous and resented her other life. Unlike Rory and Amy, who bonded by traveling together with the Doctor, Clara's relationship with Danny was doomed to fail because they didn't bridge the "TARDIS gap."
And I think she made the decision to resume traveling with the Doctor in Last Christmas because, with her anchor gone, she realized that she's just not cut out for a regular Earth-girl life.
Also, 90-year old Clara had plenty of flings in 62 years but never married...