Since all people are different, there isn't one formula for "getting into the mood". If you feel it is specifically the preparations for mikveh that wear you out, consider doing some preparations earlier in the day to avoid pressure. Showering, washing and combing your hair, and inspecting yourself, are the only preparations that must be done close to immersion. You can cut your nails and shave earlier in the day (or even the on the previous day), as long as you check that your nails are clean just before immersion. You can also bathe earlier in the day, leaving only the bare minimum for just before immersion.
Another way of alleviating pressure (though we know this is easier said than done) is a change of attitude toward the preparations. It sounds like you are preparing with a lot of strain, rather than enjoying the luxury of taking care of yourself, and giving your body full attention and treatment it deserves. If you have kids at home, and your local mikveh is nice, you should consider doing the preparations there, with no pressure. Try to enjoy the time alone and feel like you are pampering yourself and preparing for your husband rather than just preparing for immersion – this might be the only solution needed. Of course, if you find preparing at the mikveh more stressful, you can prepare in the comfort of your own home. In that case, when you get to the mikveh you can take a quick shower in warm water, comb through your hair again and inspect your body prior to immersion.
There is no obligation to have relations on the night of mikveh immersion. You should never feel like you are having relations out of obligation. However, it is a special mitzvah to have relations on the night of immersion and, more importantly, it is probably important to your husband. "Getting in the mood" sexually is, again, a purely subjective matter. Since sexual desire and arousal in women is highly dependent on their emotional state and psychological factors, you need to recognize those factors in yourself. Certainly viewing relations as an obligation would not set the right mood for most women.
We suggest thinking, planning and discussing with your husband ahead of time ways of creating the right atmosphere, which will lead to you desiring relations on mikveh night rather than feeling obligated in them. The best way to do this depends on your personality and relationship – but some suggestions could include a prepared candle-lit dinner with your favorite foods, wine, a long massage (to reward you for all your hard work in preparation), music, even going out to a restaurant after immersion, and returning home rejuvenated. These other activities might distract from the pressure of having relations, and rather give you and your husband something to look forward to later in the night, after setting the right mood with a relaxed evening. Having discussed these possibilities in advance will also lead to your husband having something to look forward to, rather than being disappointed.
Lastly, we stress again that there is no strict obligation in relations on mikveh night. There will always be times when one partner is not in the mood, despite all efforts. Consider engaging in other forms of physical contact – hugging, kissing, massaging, cuddling – which may alleviate some of the sexual tension without actually having relations.
We hope you find some of these suggestions useful!