The mind throws up junk
If we sit still for a bit we start to get a sense of what goes through our mind at any given moment - impressions, memories, words, phrases, visions and so on. A lot of this is basically junk that we discard because it is not relevant in the moment, however there is a deeper current of thought that runs through us that we do believe in - our view of the world. In order to know that this is also junk, we need to see the world through different eyes - we need to experience things differently. Then we understand that what we think is real is not real at all - indeed anything could be true if we thought about it that way. Still with me? Look at how you feel in particular situations and look at the thoughts that shape those feelings (and vice versa). The thoughts and feelings are entirely arbitrary - they are a manifestation of circumstances. You could equally be having the opposite thoughts and feelings if you'd been built that way. Indeed, if you keeping watching your mind you can observe it shifting from one particular set of thoughts into another.
Now, here's the cool bit. If you know your mind is throwing up unpleasant junk views then it is possible to have the wisdom to reject the unpleasant junk and look for constructive ways to turn it into something more beneficial to yourself. There is no underlying truth or validity to the thoughts that you believe in, they are just what has arisen for whatever reason. Normally we might buy in to what we are thinking and try to deal with it, but it is possible to realise that we could be having an entirely pleasant experience with the same set of stuff.
In short, you can make yourself happy just by knowing it is possible. We just have to see it that way.
I will perhaps try a worked example to clarify what I am babbling on about at some point