Sometimes it's just not possible to think positive. We feel rotten. We feel negative. Just the thought of "positive thinking" brings on feelings of annoyance or even anger.
This is part of being human, experiencing feelings that are what we might call "negative." They're feelings we don't want. Frustration, sadness, regret, confusion, exhaustion, guilt, fear, worry, jealousy, loneliness, grief, anger... the list goes on and on.
We often do something strange, though, when we start feel emotions that don't feel very good. Our tendency is to try to avoid them. To distract ourselves from them. We don't like these feelings; we want them to just go away. So we might pretend they're not there and find something that feels better (because we just want to feel GOOD all the time, right?). We might eat some food to comfort us, get a tall glass of wine, zone out in front of the TV or on Facebook for hours. This might work for a little while, but avoiding our emotions only works for so long. We might even seek out unhealthy ways (drugs or other harmful, addictive behaviors) to cope. If we feel persistent negative emotions that we continually ignore and push away, over time we build up stress in our bodies that can manifest as sickness. Our bodies don't lie, and will always show us what we have been not wanting to see.
So, what can be done? Well, to start, stop avoiding your feelings. If you feel something that doesn't feel good, take a moment to explore these feelings. Really wallow in the misery for a while. But don't just spin and re-live the past over and over. You want to develop what I call "an observer mentality" of your emotions. State your emotions aloud. Write them down. Release them physically by working them out of your body (running, yelling, crying, energy work, etc.). When you do this, you begin, gradually, to separate from your emotions. You might be feeling overwhelmed and confused, but you are a not a confused, overwhelmed person. You might be feeling sad and lonely, but you are not a sad and lonely person. All emotions are temporary.
And how do I know about this? Well, I write about what I personally experience. As you may know, I work pretty hard at maintaining a positive energetic vibration (focusing and dwelling in a positive mental place). But this only works part of the time. Sometimes things happen to us that we don't plan. It's part of our learning lessons in life. I had been feeling some negative emotions recently, so I decided to take some time to explore my emotions. I swam in their waters for a bit by writing about them (and I happen to have created a Processing Negative Emotions writing tool you can access here... and you have to read some nice stuff people have said about me too).
After I did this, actually while I was writing (but not just writing about my feelings in a spiral of drama, really targeting the feeling and investigating it), I started to feel that they were losing their power over me. I started to see them merely as feelings I was experiencing. And I began to see some light at the end, some hope. I felt better and even got some ideas out of it. So try it out... it might just help you reach that next level where you can begin to manifest what you want in your life...