How long have you both been trying? If more than a year you BOTH should see a fertility specialist.
To find your luteal phase we'd need to know when you ovulate. There are ovulation test kits available. Ideally, you should be try to have sex from about five days before to two days after you ovulate. Normally, this would be between about days 11 and 16 of your cycle. Day 1 is when your period starts.
There are drugs that can help regulate your period but you'd need to see a doctor. A few days change in your cycle length, either way, is not unusual.