What is wrong with my cherry tree?

I have that on the ends of most branches , the leaves are full of tiny black flies , I took off as many tips as i could and sprayed the tree with bug killer ! , hope it helps the tree , i grew it from tiny and it is a fair size now

Yes, there's something living in it.

Looks like leaf curl aphids.

My plum tree has gone like that too, and the sparrows are having a field day eating the insects (at least I think thats what they are eating), but it looks proper poorly and I doubt if we will get any fruit this year as most of it is dropping. It started off the season healthy and laden with blossom.

You could try a soapy water spray first of all, also wiping off what you can with your fingers
Could you give it a feed as well, as they could help it to resist the aphids

I hope it recovers this year - if it still looks poorly at the end of the season then a good prune could help.

that not they!

When our peach tree had some sort of wigglies hatching in it’s leaves I read that garlic planted around the base would help. Garlic was planted and the tree certainly was a lot healthier in following years and only got a touch of whatever they were. Apparently the garlic in the sap of the tree discouraged the bugs.

Watching Spring Watch this evening it was mentioned that lacewings, certain beetles and ladybirds like eating aphids.
Could you introduce some predators?
You can buy them online but whether they would stay on your tree is another matter, I think they are usually used in a greenhouse.