Anal Tip

I've been trying to do anal for a while now with my boyfriend but it never happens. I'm either to tight and it just won't go in or it goes in a little bit and starts to burn and I just can't go through with it and I just end up giving him a blowjob for the rest of the time. We've tried to stretch it out for a few minutes but it's still to tight. How do I get pass the hurting and being really tight?