Hi Creative,
Yes, this is very true about our brains. It does suck, because fun/good times are the ones we want to remember, right?
There is a trick to it. It is said that "Neurons that fire together wire together". What this means is your attention can make neurons fire, wire stronger together through a process called myelination (a substance that coats our neurons and speeds up the signals) and make the associations (bits and pieces of memories - they are never stored in one place but broken into pieces to place all around the brain; they have a theory this is done to protect us from brain trauma like an aquired brain injury) there stronger. So, next time you are in a good time, try breaking what you are experiencing into real pieces - what are you feeling, what are you smelling, what are you hearing, etc. Then think of those pieces and remember another time you experienced that.
For example (these are at random, but they need to be associated with real things that make you personally happy): Say you are at a picnic with friends. You note the breeze in your hair, reminding you of the times at the beach reading a book. You hear happy chatter from folks in the park, which reminds you of the happy chatter around the christmas dinner table. You see each one of your friends, which reminds you of a special moment shared with each one of them. You see the carrot cake your friend made, which reminds you of an Easter you got a pretty outfit. So on and so forth.
The more you tie memories to other happy memories, the stronger the brain's association pockets get. Then, when you have alone time. Meditate on one word and see where your mind takes you. If it goes to a negative, swipe left on it - don't entertain it, and steer back to a better thought.
Your attention is within your control. When left unattented to, it can take you down some troubled roads. But with your control, it will be your shield.
If you are interested in this kind of insight into your brain vs mind, have a look at cognitive neuroscience on memories and them social psychology on memories. There is also Cognitive Behavioural Therapy conversations about memories.