Tachycardia and Ivabradine

I am a 25 years old female. I've been dealing with tachycardia for the past two years. I am not sure where to post this so, bear with me.

I had several ecgs done, 24 hours Holter monitoring, and echocardiography which ruled out any heart condition. I am otherwise healthy.

My resting rate is constantly 90-115 bpm. It spikes up to 130-140 during my walks.

Cardiologist prescribed me ivabradine 5mg/2 times a day. I've been taking it for 5 days and heart resting rate has lowered at around 80 bpm on average, and 60 bpm while sleeping, which is good.

However, I feel like stopping it because I started having this feeling of breathlessness, dyspnea, mainly at rest.

Is it possible in any way that I am not used to a lower HR, hence 'needing less hair' hence the dyspnea?

I am used to palpitations and high resting rate so, I feel like I could live with it; question is, can I live with it? If so, should I limit physical activity in any way?

Thanks in advance to anyone who might have a clue.

Morphine is a bad idea.

Morphine? Mind to elaborate? I am not taking any other medication, a part from an iron supplement, nor drugs of any kind. I barely drink coffee.

I was on a very small dose of Beta-Blockers, as an ex-racing cyclist, my at rest heart beat was quite low anyway, went down to 40 on the beta-blockers, but one of the reasons (I have stopped taking them was becuase it make me feel "like my lungs no longer work". A wopman at work, without prompting, claimed she felt like "her lungs didnt work", when she was on beta-blockers. 

I tried beta blockers in the past but could not tolerate even the smallest dose because it also lowers blood pressure. I had the same feeling of breathlessness. At least I am not imagining it.

The current medication (ivabradine) is not a beta blocker - works on the 'natural pacemaker' lowering heart rate. That said, the side effects should not be worse than the actual gain.. Hopefully things will settle and I won't have dyspnea too often. Still, in a perfect world I'd rather keep the tachycardia.. Thanks for the insight.

I  have had ivabradine for a few years now and take 2.5 once a day. I feel you need to go back to your doctor and tell them how you are feeling.  I believe the heartbeat shouldn't go below 70 when taking it, so measure blood pressure before taking. Hope this helps but don't just stop taking it. 

I monitor heart rate with one of those pulsometer watches so I feel tranquil about it - it never goes under 70. I won't stop taking it but I'll surely talk to my doctor and possibly have the dosage adjusted over time. Good to know 2.5 once a day works for someone, perhaps it'll be enough for me, too. Thanks Bethy for the helpful answer!