There's a myriad of things you can do but to be honest I think until you get a handle on your self esteem issues it may all feel a bit...well, forced. I think feeling a bit happier in yourself will reflect in the confidence you have towards him and your relationship. How you do this is something only you can really figure out - for some people it comes through achievement, for other's it's about having more fun and letting go etc.

If you think you can wing it, even for a little while, maybe role play is a good suggestion - it would give you the chance to be in a second skin for a while and it may well meet the demands of your partner a bit better as well.

I have to say though, if he's openly critical of you frequently then this is something that will also need addressing. How can he expect you to be more relaxed and experimental around him if his attitude is unhelpful? Just a thought.

If this is more a case of being bored with regular sexual experiences, maybe prostate massage would be something to consider? This is something I suggest you talk about thoroughly though and do as much research as you can.

Good luck.