Mel , I have been paying attention to your posts and wanted to weigh in on this one...This is not intended to be critical in any way. You lost your husband 3 years ago and then had another relationship dissolve where you were planning a marriage within that time frame and started dating again afterwards, it's bound to catch up with you and it's normal that you may not feel happy and actually feel depress. You are blessed with so many things and those things were there prior to losing your husband but has there been something new, a purpose just for you since? Something that can be totally selfish and self centred just for you ?
Also a way to realise what you have and re learn to appreciate it? I had to tell myself over and over that my glass was half full until I started seeing it. I had to come to terms with the fact that who I was previously was no longer and to figure out what my needs were. More importantly I had to figure out who I now was and commit to figuring that out before even contemplating dating.
It may also be time to talk with your Dr and explain how you feel as it could also be a medical issue . My thyroid went out of wack and I felt some of what you have shared.
Good luck