It may be that the fire of your sexua| relationship just needs a bit of work. A good book to read is "Cupid's Poisoned Arrow", which explains how traditional sex in a long term relationship can lead to situations where there is sexua| indifference or low sexua| desire that develops between a couple.
The way to heal this issue is to have intercourse without ejaculation for both the man and the woman. The book recommends the Karezza technique, but you can also use the Tantric sex techniques.
Also, losing an erection after the start of sex is not a hinderance to having and enjoyable sex life. If both of you can let go of the goal orientation of orgasm of sex, and instead use sex as a meditation and an opportunity to feel the energies between the two of you, you will find that your experience of sex and the resulting orgasm can last more than just a few minutes of bliss, but turn into hours or even days of bliss and joy.
Another good book to read on this subject is Tantric Orgasm for Women by Diana Richardson. In this book she also explains how to enjoy sex with a man who has erectile dysfunction.