'I think he's making excuses to avoid marriage'

I have been in a relationship for a little over two years now, and I recently asked my boyfriend if he ever wanted to take this to the next stage. He responded by saying he wasn't sure yet because he needs to focus on his career. Maybe he just doesn't want to marry and is looking for an excuse. I'm confused and annoyed and am thinking about ending this relationship because of what he said.

He hasn't said anything explicitly about not wanting to marry you, so that may be an assumption. Ask him a direct question for an answer to that one. As for focusing on his career, maybe he's just being honest and needs a little more financial security before he decides to take the next step with you. Ending the relationship is not a solution unless he clearly believes there is no future for the two of you.
If he believes there is one, and you believe in it too, why not try and set a common goal you can both work towards? If that doesn't work out, at least you can both tell yourselves that you tried to make it work.

My girlfriend doesn't believe I love her. This is ridiculous because we have been together for three years and I haven't said or done anything to make her doubt me. What can I do to prove this to her? She just isn't convinced no matter what I do.
It's almost impossible to try and prove that you love someone if they don't want to believe it. Maybe what she means is she doesn't feel ‘loved' enough by you, by which I suppose she doesn't feel as if you care about some of her specific needs enough. Spend more time with her, try and focus on the little things that matter to her instead of trying to win her over with big gestures, and see if that works. She obviously doesn't hate you, considering she has been with you for three years already.