Is it normal that my mood is so erratic and changes for no reason?

For the past few years, I haven't been able to keep my head on straight. Sometimes I get really depressed and all the bad thoughts just keep rolling in - about how I have no friends, I'm lonely, I don't like my job, things will always be this way until I die whether that's sooner or later than I think.

Sometimes I plan to go out to do things, but then I get into these moods where I think, "it won't be fun anyway", "why did you ever want to do this, you're going to have a bad time", "if you go you'll just end up feeling lonely and like a loser". When this happens I'll either force myself to go and end up having a bad time or just stay home.

It's like a switch that turns on and off and it doesn't seem to matter what's going on around me. Yesterday I had a softball game where our team completely destroyed the other and we went out to celebrate. All I could do was sit at the bar by myself and I actually felt like I was going to cry. I had convinced myself that I played a really awful game and I didn't deserve to be out with the rest of the team. And that some of them might have been angry at me for holding the team back. But then thinking about things later I realized that I didn't do so bad and my mood was affecting my perception of what actually happened.

I was on medication for depression for about a year and during that time I didn't have thoughts like this. Like the thoughts about "having no friends" or being rejected never popped into my head even though they might be true. I stopped taking it though because I questioned whether it was actually helping me solve my problems, or just hiding them from myself. Or another explanation is that the "problems" that I have aren't real and are just symptoms of an illness and the pills really were helping me. It's this kind of thinking that turns my head inside out.

This morning I was feeling pretty bad and ready to cancel something I was going out to do tonight, because it was "a stupid idea" but then after having a coffee I feel a little better about going. Maybe even excited to go. Times like this it feels like I'm swinging back and forth between being depressed and helpless and happy and hopeful about the future.

I'm afraid that I'm starting to lose my grip on reality. I can't trust my own feelings or emotions. Do I feel good because good things are actually happening to me? Or is it just because I have too much caffeine, anti-depressants, or alcohol in me? When are the right times to feel happy or sad? Are there really problems in my life that are so bad that I need to make radical changes to fix them? Or do I just need to take the pills that silence those thoughts?