I was trying to conceive for 3 years with my ex and we ended up splitting up from the pressure of it. I recently got with an amazing guy and was goiing to go on the injection. I told him I already was as I really thought I couldn't get pregnant and didn't want to put him off. Well now I'm pregnant! I feel like a complete idiot for not being more careful but I honestly believed I was destined to never have children. Now I'm scared that if I go to the doctor etc they will let it slip that I wasn't on anything and my boyfriend will leave me :( what should I do?

Start being honest, whether you are pregnant or not (Find out for sure), or whether he goes or stays.

What more lies to cover old ones? That makes it worse.

Communication.

You wanted to get pregnant with your ex. Oh, well.
"we ended up splitting up from the pressure of it"

Now you got it with a rebound. Sometimes you get exactly what you ask for, but later.

Go to the doctor. Find out. Take responsibility.
Both of you.