When you're aroused your vagina will naturally lengthen/stretch to accommodate for a penis.
If you're claiming you're wet and are struggling to get 2 fingers in, I'd question how aroused you are - are you using lube as well?
Is he using fingers and slowly inserting more? Have you tried using toys? - they come in all sizes and could be great to build up the feeling of having something in you.
If you've been trying for 2 years it might be that you're unconsciously panicking or worrying which will tense your muscles. Or you're putting a lot of pressure on yourselves?
Sex should never be painful. Burning and stinging on the inside sounds similar to thrush - do you have thicker discharge? or do you feel itchy?