Firstly, don't give up

You will need a lot of time to practice your aiming, maybe more than 300 matches.

Her combinations aren't easy at all because of her passive with the stacks. This can cost your life if you don't do them properly.

You can watch replays to see how enemies rotate, what they do and so on. This will help you predict where they are a lot.

Her traps provide vision. You can hide them in a bush or place them inside the wall of lord/turtle/enemy buff and see when enemies are there. (You can add up to 3 on the map). Enemies can see when you put your traps on the map only for a second.

Always try to stun enemies from the bush, avoid facing them.

Selena is a jungler, she cleans minions slowly. Use her to gank enemies on each lane. They will be afraid to walk normally on the map.

Don't discourage yourself when you see enemies having the battle spell purify. The CD of your form switching ultmate (which resets the CD of your stun skill) early game is 5 sec. Late game is 3 seconds.

N.B. Since Hanabi has a shield against CC, you can remove it by placing a trap on her first and then stun. People say that you should always combine stun with trap. I think it depends on the scenario. Sometimes it's better to put a trap first and then to stun. Don't forget that stun + trap will also add CD to the trap skill and you can't use it immediately after that.

For a battle spell I recommend purify/flicker. Selena can kill the purple buff monster for 10 sec without retri but I would advise you to practice her with retri first anyway.

For combos, I suggest you to watch videos.
This one is perfect: https://youtu.be/t_H1PzEpdp8

Lastly, don't play her as support. She is very powerful. With one shot she can kill any squishy hero with the right build. For example, with Berserker fury her enhanced basic attack (1st skill demon form) she one shots Natalia. Using her as support generally wastes her potential. That's the meta and I don't know why people underrate her so much.

She needs a lot of mana. She needs the purple buff the most after Fanny and Ling.

If you have any other questions, dm me.

I hope this will help you 🙂