You need to find a more middle way. Rather than experience high 'highs' and low 'lows' try just being more present on a daily basis. When you are feeling low be kind to it, acknowledge it and recognise that the feelings will pass. Discontentment often comes from these rollercoaster highs and lows. When we are low we look for something to pick us up however this only lasts a short time too, then frustration and depression sets in coz inevitably it goes back to normal. Embrace these feelings as they are part of life and relationships and stop desiring constant perfection. Also try not to label relationships or people as perfect coz the fact is they aren't and that is why it is even more hard to deal with when negative thoughts set in. The moment you place expectations on something is the moment you will experience disappointment.
Look at the positive aspects, you have a safe, secure, loving relationship where you can be yourself. Sounds like the communication is great too. Now the honeymoon period is over the hard work of keeping the relationship alive begins. See it as a challenge and a chance to try new things. And don't worry your feelings are completely normal and something most people experience.
Some people are drains and some are radiators... Keep clear of the drains and hug the radiators!