It is certainly possible to try too hard. This is a good example.
An ice-cold banana fresh out of the freezer is going to cause your anal sphincter to close down to about the diameter of a cat's whisker. If you do manage to insert one, it would probably cause you to cramp up immediately.
The problem you have is one of relaxation.
First, get yourself relazed. Lie down on your bed, possibly after an enema and play with yourself anally with a buttplug, a vibrator or your tool of choice until you are able to insert something at least as large as your banana.
Second, don't use suppositories. They will cause paristalsis and cause your body to want to eject the bananas before you can even get them inside you. Just use a chemically neutral lubricant. [Not KY or any other water-based lubricant whose active ingrediant is glycerin. Glycerin causes paristalsis.]
You will need to push outward strongly [as if having a BM] in order to slide a banana in. Very strongly! You must avoid cramping down (which will just cut the banana in half) This will take practice for most people.
You will probably need a dildo or vibrator to 'pack' the sections of banana more deeply inside yourself after each insertion. You will undoubtedly need to give your body a chance to adjust to the strange feeling as you fill up.
This is not an enema and it will feel very different. The first time you try it, you may not be able to insert more than a single section of banana. With experience, you can probably insert much more, if you are patient and listen to your body.
The experience of 'letting go' while holding 5 or six bananas can be quite profound, particularly if it happens at the same time as you experience 'additional benefits'.