Hey there,
The mood swings you describe may be the result of a mood disorder such as bipolar.
You are best to seek professional help. This site is great for support and advice but dont try and get a diagnosis here.
I have recently found out I MAY be bipolar (yet to be confirmed fully). I have many of the same symptoms as yourself. My mood can change from laughter to just down right depression in a second. There are also many times I just dont feel anything at all....I feel almost heartless.
Back in my younger and single days I was very much the same with relationships. I would persue and persue and once I had them I was not interested.
I am here if you ever need to chat to someone who knows what you are going through.
But like I said....go see the professionals to get a diagnosis as quick as possible.
I have been going through this crap for too long and wish someone had diagnosed me sooner.
take care
Brad