Why do I let myself get sucked in?

Because you wish not to be lonely so your mind pushes that aside. Honestly, what you do is keep away, which is easier said then done. Trust me I know. You must keep your wits ahead of you, when you see that coming, slowly walk away until it is let go.

You need to find ways of building your confidence. Its not uncommon for people to allow others to mistreat them. Your important and need to quit allowing this to happen. Surround yourself with people who only have your best interested at heart.

you need a more constructive hobby......not a destructive one.

I understand where you are coming from. Find someone who you can be close to. Someone who you are sure will not use or hurt you. This solves your problem of loneliness and also your problem of being used. One or two close friends could really help.